- Use LEX::value_list instead of LEX::show_explain_for_thread
- Factor out common code into find_thread_by_id(ulong id)

- When JOIN::cleanup(full==TRUE) is called, the select can be in two states:
  = Right after the create_sort_index() call, when join->join_tab[0] is used to 
    read data produced by filesort().
  = After create_sort_index(), and after JOIN::reinit() calls, when 
    join->join_tab[0] has been reset to read the original data. 
- We didn't handle the second case correctly, which resulted in an attempt to free 
  the same SQL_SELECT two times. The fix is to make sure we don't double-free.

- Take into account that the optimizer may run the subquery, delete it,
  and then make a JOIN::optimize call again.

- Make Item_subselect::fix_fields() ignore UNCACHEABLE_EXPLAIN flag when deciding whether 
  the subquery item should be marked as constant.

MDEV-412: SHOW EXPLAIN: Server crashes in JOIN::print_explain on a query with inner join and ORDER BY the same column twice
- JOIN::print_explain should print pre_sort_join_tab instead of "first non-constant table".  
  The code didn't take the "non-constant" part into account.

- Fix I_S table definition for EXPLAIN output.

Fix : use attribute %config(noreplace)  for /etc/my.cnf , instead of (automatically generated) %config

- Correct the way SHOW EXPLAIN code calculates 'need_order' parameter 
  for JOIN::print_explain(). 
  The calculation is still an approximation (see MDEV entry for details) (even EXPLAIN itself
  is wrong in certain cases), but now it's a better approximation than before.

see https://mariadb.atlassian.net/browse/MDEV-11

- index_merge/intersection is unable to work on GIS indexes, because:
  1. index scans have no Rowid-Ordered-Retrieval property
  2. When one does an index-only read over a GIS index, they do not 
     get the index tuple, because index only contains bounding box of the geometry.
     This is why key_copy() call crashed.
This patch fixes #1, which makes the problem go away. Theoretically, it would 
be nice to check #2, too, but SE API semantics is not sufficiently precise to do it.

Address feedback from the second code review:
- Make SHOW EXPLAIN code convert the query string before pushing it as a warning text.   

- Address feedback from the second code review.

Now partition engine adds underlying tables to the QC and ask underlying tables engine permittion to cache the query and return result of the query.

Incorrect QC cleanup in case of table registration failure fixe.

Unified interface for myisammrg & partitioned engnes for QC.

MDEV-393. Remove --loose-pbxt=OFF/loose-skip-pbxt from bootstrapper calls to avoid "unknown parameter" warning

fix incorrect merge

- Make THD::check_killed() an inline function which makes calls to non-inline 
  functions only whern there are APC requests to be served.

- switch SHOW EXPLAIN to using an INFORMATION_SCHEMA table.

Fixed MDEV-385: mysqltest running with continue-on-error crashes on a non-SQL command producing an error 

client/mysqltest.cc:
  Added missing DBUG_VOID_RETURN

- Make SHOW EXPLAIN command be KILLable with KILL QUERY.

Thanks to Ivoz for pointing this out.

- Let QUICK_RANGE_SELECT::init_ror_merged_scan() call  quick->reset() only 
  after we've set the column read bitmaps.