Cleanup wsrep_schema and remove all references to wsrep_thd_pool

WSREP debug log levels support

* Removed all references related to wsrep_thd_pool (which was removed)

* Removed unused declarations in wsrep_schema.h

* The following would result invalid reads in
  Wsrep_schema::replay_transaction():
  ```
  frag_table->field[4]->val_str(&buf);

  Wsrep_schema_impl::end_index_scan(frag_table);
  Wsrep_schema_impl::finish_stmt(thd);
  ret= wsrep_apply_events(thd, rli, buf.c_ptr_safe(), buf.length());
  ```

  because `buf` was accessed after closing the table. The fix is to
  perform storage reads using a different THD.

* In Wsrep_schema::recover_sr_transactions(), cluster_table was opened
  for write, however it is only read here. And frag_table was opened
  for read, wereas write is potentially needed.
  Also, avoid copy caused by String::c_ptr() to zero terminate the c
  string, use c_ptr_quick instead.

Commit 22feb179 broke the build
with performance_schema disabled.

dict_col_t::same_charset(): An auxiliary function to
dict_col_t::same_format(). Determine if two non-binary string
columns have the same character set.

Global variable wsrep_debug now can be used to filter wsrep-lib messages based on debug level provided.
Type of wsrep_debug is now set to be unsigned int, so tests and configuration files changed accordingly.

When creating a field of type JSON, it will be automatically
converted to TEXT with CHECK (json_valid(`a`)), if there wasn't any
previous check for the column.

Additional things:
- Added two bug fixes that was found while testing JSON. These bug
  fixes has also been pushed to 10.3 (with a test case), but as they
  where minimal and needed to get this task done and tested, the fixes
  are repeated here.
  - CREATE TABLE ... SELECT drops constraints for columns that
    are both in the create and select part.
  - If one has both a default expression and check constraint for a
    column, one can get the error "Expression for field `a` is refering
    to uninitialized field `a`.
- Removed some duplicate MYSQL_PLUGIN_IMPORT symbols

Fixing failures of federated test on 32-bit platforms

This was developed by Aleksey Midenkov based on my design.

In the original InnoDB storage format (that was retroactively named
ROW_FORMAT=REDUNDANT in MySQL 5.0.3), the length of each index field
is stored explicitly.

Because of this, we can and now will allow instant conversion from
VARCHAR to CHAR or VARBINARY to BINARY of equal or greater size,
as well as instant conversion of TINYINT to SMALLINT to MEDIUMINT
to INT to BIGINT (while not changing between signed and unsigned).

Theoretically, we could allow changing from an unsigned integer to
a bigger unsigned integer, as well as changing CHAR to VARCHAR, but
that would require additional metadata and conversions whenever
reading old records.

Field_str::is_equal(), Field_varstring::is_equal(), Field_num::is_equal():
Return the new result IS_EQUAL_PACK_LENGTH_EXT if the table advertises
HA_EXTENDED_TYPES_CONVERSION capability and we are considering the
above-mentioned conversions.

ALTER_COLUMN_EQUAL_PACK_LENGTH_EXT: A new ALTER TABLE flag, similar
to ALTER_COLUMN_EQUAL_PACK_LENGTH but requiring conversions when
reading the data. The Field::is_equal() result IS_EQUAL_PACK_LENGTH_EXT
will map to this flag.

dtype_get_fixed_size_low(): For BINARY, CHAR and integer columns
in ROW_FORMAT=REDUNDANT, return 0 (variable length) from now on.

dtype_get_sql_null_size(): Keep returning the current size for
BINARY, CHAR and integer columns, so that in ROW_FORMAT=REDUNDANT
it will remain possible to update in place between NULL and NOT NULL
values.

btr_index_rec_validate(): Relax a CHECK TABLE length check for
ROW_FORMAT=REDUNDANT tables.

btr_cur_instant_init_low(): No longer trust fixed_len
for ROW_FORMAT=REDUNDANT tables.

We cannot rely on fixed_len anymore because the record can have shorter
length from before instant extension. Note that importing such tablespace
into earlier MariaDB versions produces ER_TABLE_SCHEMA_MISMATCH when
using a .cfg file.

In the original InnoDB storage format (which was retroactively named
ROW_FORMAT=REDUNDANT in MySQL 5.0.3), the length of each index field
is stored explicitly. Thus, we can and from now on will allow arbitrary
extension of VARBINARY and VARCHAR columns when the table is in
ROW_FORMAT=REDUNDANT.

ha_innobase::open(): Advertise a new HA_EXTENDED_TYPES_CONVERSION
capability for ROW_FORMAT=REDUNDANT tables.

Field_varstring::is_equal(): If the HA_EXTENDED_TYPES_CONVERSION
capability is advertised for the table, return IS_EQUAL_PACK_LENGTH
for any length extension.

For up to 127 bytes length, InnoDB would use 1 byte for length, and
that byte would always be less than 128. If the maximum length is
longer than 255 bytes, InnoDB would use a variable-length encoding
for the length, using 1 byte for lengths up to 127 bytes, and
2 bytes for longer lengths.

Thus, 1-byte lengths are always compatible when the maximum size
changes from less than 128 bytes to anything longer.

Field_varstring::is_equal(): Return IS_EQUAL_PACK_LENGTH also when
converting from VARCHAR less than 128 bytes to any longer VARCHAR.

10.4 make wsrep_load_data_splitting use streaming replication

Variable wsrep_load_data_splitting is deprecated and should be off
by default.

cmake-wsrep: friendly error message about missing wsrep_api.h

MDEV-17631 select_handler for a full query pushdown

Added comments and file headers for files introduced in these tasks.

This task involves the implementation for the optimizer trace.

This feature produces a trace for any SELECT/UPDATE/DELETE/,
which contains information about decisions taken by the optimizer during
the optimization phase (choice of table access method, various costs,
transformations, etc). This feature would help to tell why some decisions were
taken by the optimizer and why some were rejected.

Trace is session-local, controlled by the @@optimizer_trace variable.
To enable optimizer trace we need to write:
   set @@optimizer_trace variable= 'enabled=on';

To display the trace one can run:
   SELECT trace FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;

This task also involves:
    MDEV-18489: Limit the memory used by the optimizer trace
    introduces a switch optimizer_trace_max_mem_size which limits
    the memory used by the optimizer trace. This was implemented by
    Sergei Petrunia.

classes for Json_writer_object and Json_writer_array.
These classes will be used for the implementation
of the optimizer trace.

Without this commit the error message is:

$ cmake . -DWITH_WSREP=ON
-- Running cmake version 3.12.1
-- MariaDB 10.4.3
CMake Error at cmake/wsrep.cmake:36 (FILE):
  FILE STRINGS file
  ".../mariadb-server-10.4/wsrep-lib/wsrep-API/v26/wsrep_api.h"
  cannot be read.
Call Stack (most recent call first):
  CMakeLists.txt:147 (INCLUDE)

Having 'git config cmake.update-submodules yes' nicely updates the
build without error.

$ cmake . -DWITH_WSREP=ON
-- Running cmake version 3.12.1
-- MariaDB 10.4.3
Submodule path 'wsrep-lib': checked out 'e7d72ae7f6a6995a21d743389426a963429a1fff'
Submodule path 'wsrep-lib/wsrep-API/v26': checked out '75a5f452f2ba07b0f4a3a9a94825fccc71b27398'

Made the setting of the system variable federated_pushdown at
the default connection.

Added plugin system variable federated_pushdown.

Remove unused key_thread_bootstrap.
No more global bootstrap_error: it is return value of bootstrap() instead.

Optionally, restore the default root authentication, that is
mysql_native_password USING 'invalid' OR unix_socket

recognize new unix_socket protected accounts, and don't
scare the user into setting the password for them

Change the default authentication for root@localhost to

  IDENTIFIED VIA mysql_native_password USING 'invalid' OR unix_socket

which provides secure passwordless login, while still allowing
SET PASSWORD to work as expected.

Also create a second all-privilege account for the user that owns
datadir (and thus has full access to the data anyway).

Compile unix_socket plugin statically into the server.

cmake/wsrep: allow disabling -DWITH_WSREP=OFF

If wsrep_load_data_splitting is configured, change streaming replication
parameters internally to match the original behavior, i.e. replicate
on every 10000 rows. After load data is over, restore original
streaming replication settings.

Removed redundant wsrep_tc_log_commit().

warning.

When importing a tablespace, we must initialize dummy DEFAULT NULL
values for any instantly added columns in order to avoid a debug
assertion failure when PageConverter::update_records() invokes
rec_get_offsets(). Finally, when the operation completes, we must
evict and reload the table definition, so that the correct
default values for instantly added columns will be loaded.

ha_innobase::discard_or_import_tablespace(): On successful
IMPORT TABLESPACE, evict and reload the table definition,
so that btr_cur_instant_init() will load the correct metadata.

PageConverter::update_index_page(): Fill in dummy DEFAULT NULL values
for instantly added columns. These will be replaced upon the
completion of the operation by evicting and reloading the metadata.

row_discard_tablespace(): Invoke dict_table_t::remove_instant().
After DISCARD TABLESPACE, the table is no longer in "instant ALTER"
format, because there is no data file attached.

MDEV-18178, mariabackup script timestamp change, galera suite fixed & re-recorded tests

Fixed use of uninitialized value in wsrep_TOI_end()

The if/else was a remnant from error voting code which was not
included in the release.

Resolved the problem of forming a proper query string for FEDERATEDX.
Added test cases.

Cleanup of extra spaces.