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/slave.cc

@@ -3044,7 +3044,6 @@ err:
   change_rpl_status(RPL_ACTIVE_SLAVE,RPL_IDLE_SLAVE);
   DBUG_ASSERT(thd->net.buff != 0);
   net_end(&thd->net); // destructor will not free it, because net.vio is 0
-  close_thread_tables(thd);
   mysql_mutex_lock(&LOCK_thread_count);
   THD_CHECK_SENTRY(thd);
   delete thd;