depends on uninitialised value(s) in JOIN::save_explain_data_intern"
- Make find_best()  /* the old join optimizer code */ also use table
  condition selectivity.

Local variable table_name_buffer went out of scope while
its content was still being used by a String instance.
Moved the variable to the function scope.

fix for SP & PS

- It turns out, there are statements that will call lex_start(thd->lex) 
  after parsing has been finished. lex_start() will set lex->explain=NULL,
  which will lose the pointer to already allocated Explain_plan object.
- To get rid of this, switch to lazy creation of lex->explain.  Now, it is 
  created only when we get a part ot query plan.

- MYSQL_MULTI_DELETE_DONE probe compile failure
- show_explain_non_select.test

- When showing EXPLAIN output in the slow query log, 
  format it so that one could use grep or other tool
  to get the output.

- Save the query plan after the statement was executed so that its gets into
  the slow query log.

- Merge with 10.0-base

- Port grant_explain_non_select.{test,result} from mysql-5.6
- Per Sanja's hint, fix mysql_make_view() to take into 
  account that EXPLAIN now is not necessarily EXPLAIN SELECT.

- Fix a problem with EXPLAIN multi_table UPDATE: 
  = Do use multi_update object, because multi_update::prepare() does
    various setup, e.g. it disables index-only for the tables to be updated.
  = Protect multi_update::prepare() from being invoked multiple times. 
    If the query has subqueries, they may try to invoke it, for some reason.

- eliminate join_save_qpf() function.

The patch for bug mdev-5105 incorrectly counted conditions in nested joins.

        The emb_free_embedded_thd() has the thread-unsafe code
        so should be 'mutexed' also.

Update the SHOW EXPLAIN code to work with the 
new architecture (part#1):
Before, SHOW EXPLAIN operated on real query plan structures, 
which meant it had to check when SELECTs are created/deleted.
SELECTs would call apc_target->enable() when they got a query 
plan and disable() when their query plan was deleted.

Now, Explain data structure becomes available at once (and we
call apc_target->enable()) and then it stays until it is deleted
(when that happens, we call apc_target->disable()).

- Fix EXPLAIN INSERT DELAYED ... : do call end_delayed_insert().

        LOCK_thread_count locked when we do threads.append().

Objects of the classes Item_func_isnull and Item_func_isnotnull
must have the flag sargable set to TRUE.
Set the value of the flag sargable only in constructors of the 
classes inherited from Item_int_func.

- When INSERT catches an error, it should not attempt to send parts of 
  EXPLAIN output.

MDEV-5034:Wrong result on LEFT JOIN with a SELECT SQ or a merge view, UNION in IN subquery

Make reset null_row same as it was set in evaluate_null_complemented_join_record().

The problem was that view firlds detect null_row by not-yet-reset table.

Make IO thread check for end of event group, so that upon
disconnect at the end of an event group it can report the last
read GTID as expected. Also inject a fake Rotate event at
reconnect when skipping part of an initial event group, to
give SQL thread the correct Read_Master_Log_Pos.

Reported by Pavel Ivanov.

Mark a few PCRE CMake variables as advanced,
so the are not presented in cmake-gui by default.