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.

mysql-test/suite/rpl/r/rpl_row_implicit_commit_binlog.result

@@ -165,10 +165,6 @@ master-bin.000001	#	Table_map	#	#	table_id: # (test.tt_1)
 master-bin.000001	#	Write_rows	#	#	table_id: # flags: STMT_END_F
 master-bin.000001	#	Xid	#	#	COMMIT /* XID */
 master-bin.000001	#	Query	#	#	use `test`; SET PASSWORD FOR 'user'@'localhost'='*D8DECEC305209EEFEC43008E1D420E1AA06B19E0'
-master-bin.000001	#	Query	#	#	BEGIN
-master-bin.000001	#	Table_map	#	#	table_id: # (mysql.user)
-master-bin.000001	#	Update_rows	#	#	table_id: # flags: STMT_END_F
-master-bin.000001	#	Query	#	#	COMMIT
 -e-e-e-e-e-e-e-e-e-e-e- >> << -e-e-e-e-e-e-e-e-e-e-e-
 
 -b-b-b-b-b-b-b-b-b-b-b- >> << -b-b-b-b-b-b-b-b-b-b-b-