This task is to allow storage engines that can execute GROUP BY or
summary queries efficiently to intercept a full query or sub query from
MariaDB and deliver the result either to the client or to a temporary
table for further processing.

- Added code in sql_select.cc to intercept GROUP BY queries.
  Creation of group_by_handler is done after all optimizations to allow
  storage engine to benefit of an optimized WHERE clause and suggested
  indexes to use.
- Added group by handler to sequence engine and a group_by test suite as
  a way to test the new interface.
- Intercept EXPLAIN with a message "Storage engine handles GROUP BY"

libmysqld/CMakeLists.txt:
  Added new group_by_handler files
sql/CMakeLists.txt:
  Added new group_by_handler files
sql/group_by_handler.cc:
  Implementation of group_by_handler functions
sql/group_by_handler.h:
  Definition of group_by_handler class
sql/handler.h:
  Added handlerton function to create a group_by_handler, if the storage
  engine can intercept the query.
sql/item_cmpfunc.cc:
  Allow one to evaluate item_equal any time.
sql/sql_select.cc:
  Added code to intercept GROUP BY queries
  - If all tables are from the same storage engine and the query is
    using sum functions, call create_group_by() to check if the storage
    engine can intercept the query.
    - If yes:
       - create a temporary table to hold a GROUP_BY row or result
       - In do_select() intercept normal query execution by instead
         calling the group_by_handler to get the result
       - Intercept EXPLAIN
sql/sql_select.h:
  Added handling of group_by_handler
  Added caching of the original join tab (needed for cleanup after
  group_by handler)
storage/sequence/mysql-test/sequence/group_by.result:
  Test group_by_handler interface
storage/sequence/mysql-test/sequence/group_by.test:
  Test group_by_handler interface
storage/sequence/sequence.cc:
  Added simple group_by_engine for handling COUNT(*) and
  SUM(primary_key).  This was done as a test of the group_by_handler
  interface

Fixed compiler warnings

sql/sql_view.cc:
Ensure that merged_for_insert is properly reset
(Left of an old patch)

if we didn't write the CREATE TEMPORARY TABLE statement.

- Enable old code from one of my older changesets that didn't make into 10.0
- Fix test cased that failed as they expected DROP TEMPORARY TABLE in the log.

Remove debug printout that was accidentally left in place

- Turning get_mm_tree_for_const() from a static function into
  a protected method in Item.
- Adding a new class Item_bool_func2_with_rev, for the functions and operators
  that have a reverse function and can use the range optimizer for
  to optimize "value OP field" as "field REV_OP value". Deriving
  Item_bool_rowready_func2 and Item_funt_spatial_rel from the new class.
- Removing Item_bool_func2::have_rev_func().

- Item_bool_func::get_func_mm_tree()
- Item_bool_func::get_mm_parts()
- Item_bool_func::get_ne_mm_tree()

from Item_bool_func::get_mm_leaf() into Field_xxx::can_optimize_range().
This reduces the total amount of virtual calls. Also, it's a prerequisite
change for the pluggable data types.

MDEV-8523: InnoDB: Assertion failure in file buf0buf.cc line 5963 (Failing assertion: key_version == 0 || key_version >= bpage->key_version)

Item_ref::val_bool_result().

Check if the engine is supported/allowed before replicating the
statement.

Fixed race condition in code filling INFORMATION_SCHEMA.PROCESSLIST.INFO_BINARY.
When loading query string/length of another connection one must have
LOCK_thd_data locked.

The patch for MDEV-8871 also fixed the problem reported in MDEV-657.
Adding the test case from the bug report.

MDEV-8873 Wrong field type or metadata for LEAST(int_column,string_column)

Analysis: When a page is read from encrypted table and page can't be
decrypted because of bad key (or incorrect encryption algorithm or
method) page was incorrectly left on buffer pool.

Fix: Remove page from buffer pool and from pending IO.

MDEV-8875 Wrong metadata for MAX(CAST(time_column AS DATETIME))

Item_func_hybrid_field_type did not return correct field_type(), cmp_type()
and result_type() in some cases, because cached_result_type and
cached_field_type were set in independent pieces of the code and
did not properly match to each other.
Fix:
- Removing Item_func_hybrid_result_type
- Deriving Item_func_hybrid_field_type directly from Item_func
- Introducing a new class Type_handler which guarantees that
  field_type(), cmp_type() and result_type() are always properly synchronized
  and using the new class in Item_func_hybrid_field_type.

Added error handling to crash recovery so that we stop instead of
asserting.

Alloc creating crypt_data to empty page at crash recovery and redo.

MDEV-8852 Implicit or explicit CAST from MAX(string) to INT,DOUBLE,DECIMAL does not produce warnings

Added per-table boolean IETF_QUOTES variable to CSV storage engine. It allows to
enable IETF-compatible parsing of embedded quote and comma characters. Disabled
by default.

This patch is based on Percona revision:
https://github.com/percona/percona-server/commit/b32fbf02766f8db00f9ff4e6dc05a1df97c10ff7

Note that original patch adds server variable, while this patch adds per-table
variable.

Addendum:
* Before calling THD::init_for_queries(), flip the current_thd to wsrep
thread so that memory gets allocated for the right THD.
* Use wsrep_creating_startup_threads instead of plugins_are_initialized
as the condition for the execution of THD::init_for_queries() within
start_wsrep_THD(), as use of latter could still leave some room for
race.

Problem:
When mysqld starts as a galera node, it creates 2 system threads
(applier & rollbacker) using start_wsrep_THD(). These threads are
created before plugin initialization (plugin_init()) for SST methods
like rsync and xtrabackup.

The threads' initialization itself can proceed in parallel to mysqld's
main thread of execution. As a result, the thread initialization code
(start_wsrep_THD()) can end up accessing some un/partially initialized
structures (like maria_hton, in this particular case) resulting in
segfault.

Solution:
Fixed by calling THD::init_for_queries() (which accesses maria_hton)
only after the plugins have been initialized.

MDEV-8819: Failing assertion: block->page.space == page_get_space_id(page_align(ptr)) in file buf0buf.cc line 2551

Add error handling on page reading and do not try to access empty pages.

Merge downstream Debian packaging (MDEV-6284)

The fix for MDEV-8466 earlier fixed MDEV-8635 as well. Adding a test only.

- Add EXPLAIN/ANALYZE FORMAT=JSON handling for a few special cases.