The problem was when a GTID event was part of a group commit, and so contained
a commit id. The code that replaces GTID with a BEGIN event for old slaves did
not correctly handle this case.

Fix the code so that the GTID with commit id can also be properly replaced
with a BEGIN query event. The extra two bytes are in the BEGIN event replaced
with a dummy, empty time zone string.

Older master has no GTID events, so such events are not available for
deciding on scheduling of event groups and so on.

With this patch, we run such events from old masters single-threaded, in the
sql driver thread.

This seems better than trying to make the parallel code handle the data from
older masters; while possible, this would require a lot of testing (as well as
possibly some extra overhead in the scheduling of events), which hardly seems
worthwhile.

With parallel replication, there can be any number of events queued on
in-memory lists in the worker threads.

For normal STOP SLAVE, we want to skip executing any remaining events on those
lists and stop as quickly as possible.

However, for START SLAVE UNTIL, when the UNTIL position is reached in the SQL
driver thread, we must _not_ stop until all already queued events for the
workers have been executed - otherwise we would stop too early, before the
actual UNTIL position had been completely reached.

The code did not handle UNTIL correctly, stopping too early due to not
executing the queued events to completion. Fix this, and also implement that
an explicit STOP SLAVE in the middle (when the SQL driver thread has reached
the UNTIL position but the workers have not) _will_ cause an immediate stop.

MDEV-5748 Assertion `status_var.memory_used == 0' fails on disconnect after opening an OQGRAPH table

make sure MY_THREAD_SPECIFIC is not set for memroot that can be
transferred between threads

MDEV-5674 Valgrind warnings "Conditional jump or move depends on uninitialised value" in create_sort_index with small sort_buffer_size

*found_rows wasn't initialized when filesort() failed
(it didn't matter, but valgrind was unhappy)

use the Alter_inplace_info::ALTER_COLUMN_OPTION flag if
field/column flags were altered.

change ha_example to use check_if_supported_inplace_alter() instead
of obsolete check_if_incompatible_data()

parser: error out on empty role names

don't forget to initialize ORDER::fast_field_copier_setup

Fix ha_table_exists() to take discovery into account correctly.
It must be able to discover both table existence (when no frm is
found) and table non-existance (when frm was found).

apply the upstream patch

Disable query cache for OQGRAPH

Add a check for binlog open already during the parsing, so that
BINLOG_GTID_POS() can return the error early and not delay it
until execution.

update test results

(assert added by MySQL verified that strnxfrm can only *increase*
the string length if from == to, and the latter is a random
decision made by individual items and String::realloc).
  
Fix it by avoiding the memcpy in the first place.

Clean up and improve the parallel implementation code, mainly related to
scheduling of work to threads and handling of stop and errors.

Fix a lot of bugs in various corner cases that could lead to crashes or
corruption.

Fix that a single replication domain could easily grab all worker threads and
stall all other domains; now a configuration variable
--slave-domain-parallel-threads allows to limit the number of
workers.

Allow next event group to start as soon as previous group begins the commit
phase (as opposed to when it ends it); this allows multiple event groups on
the slave to participate in group commit, even when no other opportunities for
parallelism are available.

Various fixes:

 - Fix some races in the rpl.rpl_parallel test case.

 - Fix an old incorrect assertion in Log_event iocache read.

 - Fix repeated malloc/free of wait_for_commit and rpl_group_info objects.

 - Simplify wait_for_commit wakeup logic.

 - Fix one case in queue_for_group_commit() where killing one thread would
   fail to correctly signal the error to the next, causing loss of the
   transaction after slave restart.

 - Fix leaking of pthreads (and their allocated stack) due to missing
   PTHREAD_CREATE_DETACHED attribute.

 - Fix how one batch of group-committed transactions wait for the previous
   batch before starting to execute themselves. The old code had a very
   complex scheduling where the first transaction was handled differently,
   with subtle bugs in corner cases. Now each event group is always scheduled
   for a new worker (in a round-robin fashion amongst available workers).
   Keep a count of how many transactions have started to commit, and wait for
   that counter to reach the appropriate value.

 - Fix slave stop to wait for all workers to actually complete processing;
   before, the wait was for update of last_committed_sub_id, which happens a
   bit earlier, and could leave worker threads potentially accessing bits of
   the replication state that is no longer valid after slave stop.

 - Fix a couple of places where the test suite would kill a thread waiting
   inside enter_cond() in connection with debug_sync; debug_sync + kill can
   crash in rare cases due to a race with mysys_var_current_mutex in this
   case.

 - Fix some corner cases where we had enter_cond() but no exit_cond().

 - Fix that we could get failure in wait_for_prior_commit() but forget to flag
   the error with my_error().

 - Fix slave stop (both for normal stop and stop due to error). Now, at stop
   we pick a specific safe point (in terms of event groups executed) and make
   sure that all event groups before that point are executed to completion,
   and that no event group after start executing; this ensures a safe place to
   restart replication, even for non-transactional stuff/DDL. In error stop,
   make sure that all prior event groups are allowed to execute to completion,
   and that any later event groups that have started are rolled back, if
   possible. The old code could leave eg. T1 and T3 committed but T2 not, or
   it could even leave half a transaction not rolled back in some random
   worker, which would cause big problems when that worker was later reused
   after slave restart.

 - Fix the accounting of amount of events queued for one worker. Before, the
   amount was reduced immediately as soon as the events were dequeued (which
   happens all at once); this allowed twice the amount of events to be queued
   in memory for each single worker, which is not what users would expect.

 - Fix that an error set during execution of one event was sometimes not
   cleared before executing the next, causing problems with the error
   reporting.

 - Fix incorrect handling of thd->killed in worker threads.

Removed repetative calls of virtual functions.
Removed check of posibility just copy field for every record.

Ported fix for MySQL BUG#51861.

- Change the default flag value to ON.
- Update the testcases to be run extended_keys=ON:
  = trivial test result updates
  = If extended_keys setting makes a difference for a testcase, run the testcase 
    with extended_keys=off. There were only a few such cases
- Update to vcol_select_innodb looks like a worse plan but it will be gone in 10.0.