Apply fix suggested by Igor:
- When eliminate_item_equal() generates pair-wise equalities from a 
  multi-equality,  do generate a "bridge" equality between the first 
  field inside SJM nest and the field that's first in the overall multi-equality.

MDEV-5113 Wrong result (extra row) and valgrind warnings in Item_maxmin_subselect::any_value on 2nd execution of PS with SELECT subquery

When setting Item_func_not_all::test_sum_item or Item_func_not_all::test_sub_item,
reset the other one to NULL - they can never be set both. When a PS is reexecuted,
different executions might be optimized differently and a wrong test_su*_item
might stay set from the previous execution.

- MIN/MAX optimizer does a check whether a "field CMP const" comparison uses a constant 
  that's longer than the field it is compared to.  Make this check only for string columns,
  also compare character lengths, not byte lengths.

allocate thd_proc_info string in thd memroot, not on the stack, so that it
won't be overwritten while another thread might be printing it

sort Com_ counters. No simple fix for Binlog_ variables.

MDEV-5153: Server crashes in Item_ref::fix_fields on 2nd execution of PS with LEFT JOIN and MERGE view or SELECT SQ

1. Transformation of row IN subquery made the same as single value.
2. replace_where_subcondition() made working on several layers of OR/AND because it called on expression before fix_fields().

(and valgrind warnings)

* move thd userstat initialization to the same function
  that was adding thd userstat to global counters.
* initialize thd->start_bytes_received in THD::init
  (when thd->userstat_running is set)

Singular Item_equal support added.

The problem was that during constant table substitution Item_equal become containing only one constant which was not supported internally.

mysql_upgrade should do --skip-write-binlog by default

by convention query execution time should be 0 if its start_time is 0
(this was lost when fixing MDEV-4578)

port mysqld_safe numa extensions from percona-server: --flush-caches and --numa-interleave

1. use an appropriate errno code
2. put a comma between the errno and the error message text

before running mysql_install_db check for the existence
of $datadir/mysql, not simply $datadir ($datadir might be mounted on
a separate device - exists, but empty)

fix tests where a path was used as a regex.
revert changes to sys_vars.character_sets_dir_basic - we don't run it on windows anyway

quote the path when using it in a regex - the path might contain wildcards (e.g. +)

expand fullhostname inside the string, to have an empty string, not nothing,
when fullhostname is not defined

The loop in the binary search in remove_status_vars() was
incorrectly implemented and could continue infinitely in some cases.
Rewrote the binary search code.

when our package is to replace "mysql", it has both provide and obsolete it

Modified according to the review comment

An addition to fix for MDEV-5205, fixes server crash on shutdown.

Thread groups are destroyed asynchronously, that is kill server
thread sends shutdown request to all thread groups without waiting
for compeltion.

It means all_groups array must not be freed until all thread groups
are down. This patch suggests that all_groups is freed when last
thread group is destroyed.

Note 1: threadpool code doesn't surround atomic ops with atomic locks,
thus no locks for shutdown_group_count.
Note 2: this patch preserves old behaviour, but we may need to wait
until all thread groups are down before returning from tp_end().

include/CMakeLists.txt:
  Added handler_ername.h
include/handler_ername.h:
  Names of handler error messages
mysql-test/extra/binlog_tests/binlog.test:
  Test using handler error names
mysql-test/r/mysqltest.result:
  Update for new error message
mysql-test/t/auto_increment.test:
  Test using handler error names
mysql-test/t/auto_increment_ranges.inc:
  Test using handler error names
mysql-test/t/replace.test:
  Test using handler error names

storage/xtradb/handler/ha_innodb.cc:
  Fixed wrong last argument to buf_LRU_free_block(): The LRU chain is already locked by this function.

- thread_pool_size command line option upper limit increased to 100 000
  (same as for max_connections)
- thread_pool_size system variable upper limit is maximum of 128 or
  the value given at command line
- thread groups are now allocated dynamically

Different limit for command line option and system variable was done to
avoid additional mutex for all_groups and threadpool_max_size.

  
  There was 2 problems:
  1) coping/moving of the same type (usually casting) as sizeof() (solved in different ways depends on the cause);
  2) using 'const' in SSL_CTX::getVerifyCallback() which return object (not reference) and so copy of the object will be created and 'const' has no sens.

Analysis:
st_select_lex_unit::prepare() computes can_skip_order_by as TRUE.
As a result join->prepare() gets called with order == NULL, and
doesn't do name resolution for the inner ORDER clause. Due to this
the prepare phase doesn't detect that the query references non-exiting
function and field.
  
Later join->optimize() calls update_used_tables() for a non-resolved
Item_field, which understandably has no Field object. This call results
in a crash.

Solution:
Resolve unnecessary ORDER BY clauses to detect if they reference non-exising
objects. Then remove such clauses from the JOIN object.