MDEV-17631 select_handler for a full query pushdown

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

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

Added plugin system variable federated_pushdown.

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

Cleanup of extra spaces.

MDEV-17631 select_handler for a full query pushdown

Interfaces + Proof of Concept for federatedx with test cases.

The interfaces have been developed for integration of ColumnStore engine.

test a multi-auth with a missing auth plugin on the client

update (and simplify) plugin_auth.test to match

introduce the syntax

... IDENTIFIED { WITH | VIA }
      plugin [ { USING | AS } auth ]
 [ OR plugin [ { USING | AS } auth ]
 [ OR ... ]]

Server will try auth plugins in the specified order until the first
success. No protocol changes, server uses the existing "switch plugin"
packet.

The auth chain is stored in json as

  "auth_or":[{"plugin":"xxx","authentication_string":"yyy"},
             {},
             {"plugin":"foo","authentication_string":"bar"},
            ...],
  "plugin":"aaa", "authentication_string":"bbb"

Note:
* "auth_or" implies that there might be "auth_and" someday;
* one entry in the array is an empty object, meaning to take plugin/auth
  from the main json object. This preserves compatibility with
  the existing mysql.global_priv table and with the mysql.user view.
  This entry is preferrably a mysql_native_password plugin for a
  non-empty mysql.user.password column.

SET PASSWORD is supported and changes the password for the *first*
plugin in the chain that has a notion of a "password"

add DEPRECATED() warning for variables with "deprecated" in the
help text. Remove redundant initializers.

if auth_root_socket is set, root user should be using unix_socket

Followup for 4abb8216

Revert the side effect of 7c40996c.
Do not convert password hash to its binary representation when a user
entry is loaded. Do it lazily on the first authenticatation attempt.

As a collateral - force all authentication plugins to follow the
protocol and read_packet at least once before accessing info->username
(username is not available before first client handshake packet is read).

Fix PAM and GSSAPI plugins to behave.

add tests

Add a system variable spider_slave_trx_isolation.
- spider_slave_trx_isolation
  The transaction isolation level when Spider table is used by slave SQL thread.
  -1 : OFF
   0 : READ UNCOMMITTED
   1 : READ COMMITTED
   2 : REPEATABLE READ
   3 : SERIALIZABLE
  The default value is -1

Miscellaneous Spider typos

Change default value of the followings
quick_mode 0 -> 3
quick_page_size 100 -> 1024

Add the following parameter for limiting result page size by byte
- quick_page_byte(qpb)
  Number of bytes in a page when acquisition one by one.
  When quick_mode is 1 or 2, Spider stores at least 1 record even if
  quick_page_byte is smaller than 1 record. When quick_mode is 3,
  quick_page_byte is used for judging using temporary table.
  That is given to priority when server parameter spider_quick_page_byte
  is set.
  The default value is 10485760

Fix "out of sync" issue at using quick_mode = 1 or 2

The fields of the temporary table were not created in create_tmp_table function. Because item->const_item() was true. But the temporary tables that is created by Spider are always used all columns. So Spider should call create_tmp_table function with TMP_TABLE_ALL_COLUMNS flag.

Store original charset during client authentication, and restore it for
COM_RESET_CONNECTION

The default value for eq_range_index_dive_limit is set to 10

No need to lowercase table names on case-sensitive file systems, as the
cache won't contain the 'lowercased' table anyway. And it prevents the
UPPERCASE.frm from being deleted.

Problem:
=======
Mariabackup incremental prepare creates new tablespace when it encounter
new tablespace. It sets the intial size as FIL_IBD_FILE_INITIAL_SIZE (4).
But while applying redo log, it tries to access 5th page and then
it leads to out of tablespace error.

Fix:
===
While parsing the redo log record, track FSP_SIZE in recv_spaces for the
respective space id. Assign the recv_size for the tablespace when it
is loaded. Extend the tablespace depends on recv_size while applying
the redo log record.

- Added retry logic if validation of first page fails with checksum
mismatch.

Do not try to write ER_SHUTDOWN error message to socket, when it is forcefully closed by the shutdown.
This will avoid the race condition (attempt to write to closed socket,  if connection shuts down by itself).

MDEV-18426: Most of the mtr tests in the galera_3nodes suite fail

add simplified slave_trx_isolation.test

add simplified quick_mode.test

Add a system variable spider_slave_trx_isolation.
- spider_slave_trx_isolation
  The transaction isolation level when Spider table is used by slave SQL thread.
  -1 : OFF
   0 : READ UNCOMMITTED
   1 : READ COMMITTED
   2 : REPEATABLE READ
   3 : SERIALIZABLE
  The default value is -1

Miscellaneous Spider typos

Change default value of the followings
quick_mode 0 -> 3
quick_page_size 100 -> 1024

Add the following parameter for limiting result page size by byte
- quick_page_byte(qpb)
  Number of bytes in a page when acquisition one by one.
  When quick_mode is 1 or 2, Spider stores at least 1 record even if
  quick_page_byte is smaller than 1 record. When quick_mode is 3,
  quick_page_byte is used for judging using temporary table.
  That is given to priority when server parameter spider_quick_page_byte
  is set.
  The default value is 10485760

Fix "out of sync" issue at using quick_mode = 1 or 2

The fields of the temporary table were not created in create_tmp_table function. Because item->const_item() was true. But the temporary tables that is created by Spider are always used all columns. So Spider should call create_tmp_table function with TMP_TABLE_ALL_COLUMNS flag.