A pre-requisite patch for the fix for Bug#52044.

This patch also fixes Bug#55452 "SET PASSWORD is
replicated twice in RBR mode".

The goal of this patch is to remove the release of 
metadata locks from close_thread_tables().
This is necessary to not mistakenly release
the locks in the course of a multi-step
operation that involves multiple close_thread_tables()
or close_tables_for_reopen().

On the same token, move statement commit outside 
close_thread_tables().

Other cleanups:
Cleanup COM_FIELD_LIST.
Don't call close_thread_tables() in COM_SHUTDOWN -- there
are no open tables there that can be closed (we leave
the locked tables mode in THD destructor, and this
close_thread_tables() won't leave it anyway).

Make open_and_lock_tables() and open_and_lock_tables_derived()
call close_thread_tables() upon failure.
Remove the calls to close_thread_tables() that are now
unnecessary.

Simplify the back off condition in Open_table_context.

Streamline metadata lock handling in LOCK TABLES 
implementation.

Add asserts to ensure correct life cycle of 
statement transaction in a session.

Remove a piece of dead code that has also become redundant
after the fix for Bug 37521.

sql/sql_class.cc

@@ -29,7 +29,6 @@
 #include "sql_priv.h"
 #include "unireg.h"                    // REQUIRED: for other includes
 #include "sql_class.h"
-#include "lock.h"      // unlock_global_read_lock, mysql_unlock_tables
 #include "sql_cache.h"                          // query_cache_abort
 #include "sql_base.h"                           // close_thread_tables
 #include "sql_time.h"                         // date_time_format_copy
@@ -1817,12 +1816,6 @@ bool select_send::send_eof()
   */
   ha_release_temporary_latches(thd);
 
-  /* Unlock tables before sending packet to gain some speed */
-  if (thd->lock && ! thd->locked_tables_mode)
-  {
-    mysql_unlock_tables(thd, thd->lock);
-    thd->lock=0;
-  }
   /* 
     Don't send EOF if we're in error condition (which implies we've already
     sent or are sending an error)