https://mariadb.atlassian.net/browse/MDEV-28
  
This task implements a new clause LIMIT ROWS EXAMINED <num>
as an extention to the ANSI LIMIT clause. This extension
allows to limit the number of rows and/or keys a query
would access (read and/or write) during query execution.

make the test to require sphinx 2.0.4 or later

Fixed memory leaks and compiler warnings in ha_sphinx.cc
Added HA_MUST_USE_TABLE_CONDITION_PUSHDOWN so that an engine can force index condition to be used

mysql-test/suite/sphinx/sphinx.result:
  Added testing of pushdown conditions and sphinx status variables.
mysql-test/suite/sphinx/sphinx.test:
  Added testing of pushdown conditions and sphinx status variables.
mysql-test/suite/sphinx/suite.pm:
  Print version number if sphinx version is too old.
sql/handler.h:
  Added HA_MUST_USE_TABLE_CONDITION_PUSHDOWN so that an engine can force index condition to be used
sql/sql_base.cc:
  Added 'thd' argument to check_unused() to be able to set 'entry->in_use' if we call handler->extra().
  This was needed as sphinx (and possible other storage engines) assumes that 'in_use' is set if handler functions are called.
sql/sql_select.cc:
  Test if handler is forcing pushdown condition to be used.
storage/sphinx/ha_sphinx.cc:
  Updated to version 2.0.4
  Fixed memory leaks and compiler warnings.
storage/sphinx/ha_sphinx.h:
  Updated to version 2.0.4
storage/sphinx/snippets_udf.cc:
  Updated to version 2.0.4

The function create_hj_key_for_table() that builds the descriptor of
the hash join key to access a table of a materialized subquery must
ignore any equi-join predicate depending on the tables not belonging
to the subquery.

This is needed as last log entry may be a DDL that is not processed and then a table may be left in 'not properly closed state' even if information is correct in it.

mysql-test/README.suites:
  removed. it was turned into a manual, eventually
mysql-test/include/default_mysqld.cnf:
  use normal environment variables instead of magic @OPT.vardir
mysql-test/lib/My/ConfigFactory.pm:
  use normal environment variables instead of magic @OPT.vardir
mysql-test/mysql-test-run.pl:
  initialize environment variables before parsing the first my.cnf template - it may refer to them
mysql-test/suite/sphinx/my.cnf:
  use normal environment variables instead of magic @OPT.vardir

Problem is that subquery execution can't be called during prepare/optimize phase.

Also small fix for subquery test suite.

This bug in the function JOIN::drop_unused_derived_keys() could
leave the internal structures for a materialized derived table
in an inconsistent state. This led to a not quite correct EXPLAIN
output when no additional key had been created to access the table.
It also may lead to more serious consequences: so, the test case
added with this fix caused a crash in mariadb-5.5.20.  

mysql-test/include/rpl_udf.inc:
  supppression should be added after master-slave.inc to affect both a master and a slave
mysql-test/suite/rpl/t/rpl_udf.test:
  supppression should be added after master-slave.inc to affect both a master and a slave

mysql-test/include/have_xtradb.combinations:
  xtradb tests need this I_S table
mysql-test/lib/mtr_cases.pm:
  fix for .deb and .rpm installations

 - use thd->in_active_multi_stmt_transaction() for
   @@in_transaction, not THD flag directly
 - use common error_if_in_trans_or_substatement() function for
   all 4 binlog variables that cannot be changed inside
   statement or transaction.

This bug appeared after the patch for bug 939009 that in the
function merge_key_fields forgot to reset a proper value for
the val field in the result of the merge operation of the key
field created for a regular key access and  the key field
created to look for a NULL key.

Adjusted the results of the test case for bug 939009 that
actually were incorrect. 

Otherwise XA crashes after FLUSH STATUS as log page size suddenly becomes 0.

Fixed lp:925377 "Querying myisam table metadata while 'alter table..enable keys' is running may corrupt the table"
Fixed wrong mutex order bug in Aria when flush_log_for_bitmap() was called when table is not yet marked for change.

include/my_base.h:
  Added flag that table is opened only for status
mysql-test/r/myisam-big.result:
  Test case for lp:925377
mysql-test/t/myisam-big.test:
  Test case for lp:925377
sql/sql_base.cc:
  If thd->version == 0 (happens only when we are opening a table that is flushed under  MYSQL_LOCK_IGNORE_FLUSH), open the table in HA_OPEN_FOR_STATUS mode
storage/maria/ma_bitmap.c:
  Fixed wrong mutex order bug in Aria when flush_log_for_bitmap() was called when table is not yet marked for change.
storage/maria/ma_dbug.c:
  Ignore last_version <= 1 as these are either flushed or only opened for status
storage/maria/ma_open.c:
  Use last_version=1 as a marker that table was opened with HA_OPEN_FOR_STATUS.
  In this case we just open a new version of the table in read only mode.
storage/myisam/mi_create.c:
  Update prototype
storage/myisam/mi_dbug.c:
  Ignore last_version <= 1 as these are either flushed or only opened for status
storage/myisam/mi_open.c:
  Use last_version=1 as a marker that table was opened with HA_OPEN_FOR_STATUS.
  If HA_OPEN_FOR_STATUS is used, we will not assert if there is an old not-to-be-used version of the table existing.
  In this case we just open a new version of the table in read only mode.
storage/myisam/myisamdef.h:
  Updated prototype

make sure that stored routines are evaluated (that is, de facto - cached) in convert_const_to_int().
revert the fix for lp:806943 because it cannot be repeated anymore.
add few tests for convert_const_to_int()