The transaction isolation levels READ COMMITTED and READ UNCOMMITTED
should behave similarly to the old deprecated setting
innodb_locks_unsafe_for_binlog=1, that is, avoid acquiring gap locks.

row_search_mvcc(): Reduce the scope of some variables, and clean up
the initialization and use of the variable set_also_gap_locks.

The parameter innodb_log_checksums that was introduced in MariaDB 10.2.2
via mysql/mysql-server@af0acedd885eb7103e319f79d25fda7386ef1506
does not make much sense. The original motivation of introducing this
parameter (initially called innodb_log_checksum_algorithm in
mysql/mysql-server@22ba38218e1d76c24f69b5a5595ad3bf5933acb0)
was that the InnoDB redo log used the slow and insecure innodb algorithm.
With hardware or SIMD accelerated CRC-32C, there should be no reason to
allow checksums to be disabled on the redo log.

The parameter innodb_encrypt_log already implies innodb_log_checksums=ON.

Let us deprecate the parameter innodb_log_checksums and always compute
redo log checksums, even if innodb_log_checksums=OFF is specified.

An upgrade from MariaDB 10.2.2 or later will only be possible after
using the default value innodb_log_checksums=ON. If the non-default
value innodb_log_checksums=OFF was in effect when the server was shut down,
a log block checksum mismatch will be reported and the upgraded server
will fail to start up.

maria_open(): Initialize share->mode=0 for S3

The test result was affected by
commit 007f68c3
but the adjustment was inadvertently omitted.

To get a stacktrace in a program like aria_chk, one only have to do:

#include <my_stacktrace.h>

call my_init_stacktrace(1) in main().

A read-only storage engine that stores it's data in (aws) S3

To store data in S3 one could use ALTER TABLE:
ALTER TABLE table_name ENGINE=S3

libmarias3 integration done by Sergei Golubchik
libmarias3 created by Andrew Hutchings

Removed not needed table renames when doing ALTER TABLE when engine
changes and both of the following is true:
- Either new or old engine does not store the table in files
- Neither old or new engine uses files from another engine

We also skip renames when ALTER TABLE does an explicit rename

This improves performance, especially for engines where rename is
a slow operation (like the upcoming S3 engine)

Discoverred to be needed by the upcoming S3 engine

Reason for the change was that ha_notify_table_changed() was done
after table open when .frm had been replaced, which caused failure
in engines that checks on open if .frm matches the engines table
definition.

Other changes:
- Remove not needed open/close call at end of inline alter table.
  Some test that depended on the table beeing in the table cache after
  ALTER TABLE had to be updated.

This was done to match the naming of the ALTER #sql-#-# tmp files

No need to initialize socket_accept, it always gets assigned a value.

Pass addr_len directly to accept().

Part of MDEV-19515 - Improve connect speed

Removed LOCK_connection_count.

Removed duplicate denied_connections and connection_errors_max_connection
increment from create_new_thread(). Another increment done by
CONNECT::close_with_error().

Simplified away CONNECT::thread_count_incremented. Now close_with_error()
is always called with connection_count incremented.

Part of MDEV-19515 - Improve connect speed

cached_thread_count can handle it alone.

Added "fast path": when thread is being added to the cache and there's
queued connection request, serve it immediately.

Part of MDEV-19515 - Improve connect speed

It mostly duplicates code of do_handle_one_connection(). Move it there,
so that it is easier to simplify it in future.

Moved PSI_CALL_delete_current_thread()/_db_pop_() out of mutex. Call them
consistently in all cases, unlike before:
- called for reused threads
- called for flushed threads
- called for threads that go down after 5 minutes timeout
- not called for threads attempting to cache themselves during flush
- not called for threads attempting to cache themselves when cache is full

Part of MDEV-19515 - Improve connect speed

Code properly integrated into callers instead.

do_handle_one_connection(): no need to reset thd and thd->thread_stack
as they're not updated by cache_thread().

Part of MDEV-19515 - Improve connect speed

It was meaningful only for one-thread-per-connection scheduler anyway,
so call init_new_connection_handler_thread() directly from
handle_one_connection().

It was somewhat harmful for no-threads scheduler, because it'd attempt to
detach main thread on AIX_3_2 and Siemens unix (and DEC OSF/1 3.2 too).
Also it does duplicate my_thread_init() call, which may produce a warning
under EXTRA_DEBUG_THREADS.

start_wsrep_THD() shouldn't have called it at all: it already detached
and called my_thread_init().

Part of MDEV-19515 - Improve connect speed

Gives ~10% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed

Gives ~3% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed

Rather than iterating global plugin collection, iterate THD local
collection. Removes two LOCK_plugin locks per connection.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in rocksdb_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Removed transaction objects cleanup from rocksdb_done_func(). As we lock
plugin properly, there must be no transaction objects during RocksDB
shutdown.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in sphinx_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in mrn_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Do not reset THD ha_data in spider_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in ha_federatedx::disconnect(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

It was wrong anyway, as it may not get updated by the time created thread
reads it. And it was 0 for cached threads.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

It may be beneficial for slow-path, where new thread is to be created.
However it is harmful for fast-path, where thread cache can fulfil
request: it caused 2 extra loads of write-contended variables.

Also shrink variables scope.

Part of MDEV-19515 - Improve connect speed