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

The reasons for failures to initialize a user collation defined
in Index.xml are now correctly reported                                  
(in SHOW WARNINGS and in the server error log) if the COLLATE clause
is used with an expression, e.g.:

  SELECT <exrp> COLLATE <collation name> FROM t1;

Previously, the failure reasons were reported only by COLLATE clauses
in SET NAMES and in DDL statements, e.g.:

  SET NAMES utf8 COLLATE utf8_xxx_ci;
  CREATE TABLE t1 (a VARCHAR(1) CHARACTER SET utf8 COLLATE utf8_xxx_ci);

- 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().

When we open merge children open error is normally handled early during
open phase. But there are two exceptions when error is handled later during
attach phase: CHECK/REPAIR TABLE and tables added by the pre-locking code.

The latter case wasn't considered by assertion in the merge code. This
assertion is corrected. Note that in MySQL-5.6 this assertion is removed.

        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.