- Make mysql_select() return error when the query was killed.

MDEV-5141: Failing assertion: ib_table->stat_initialized in file ha_innodb.cc line 11042 on concurrent ALTER and SELECT from I_S

Analysis: After ALTER TABLE the table statistics needs to be rebuilt and therefore stat_initialized is set false.  It will be rebuilt when the table is loaded again and table is closed when alter table is completed. However, during alter table table could be used by concurrent SELECT from I_S. Therefore, we need to rebuild transient table statistics meanwhile until table can be reloaded.

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.