WL#5403: LOCK TABLES t1 READ, t_does_not_exist WRITE;

Allow LOCK TABLES statement to lock non-existing tables for WRITE. 
That would make possible RENAME under LOCK TABLES. It would also allow us to fix
the broken semantics of DROP and ALTER TABLE RENAME in LOCK TABLES mode, which
currently also drop the table lock on the renamed/dropped table.

Affected bugs
=============
BUG#22246 "Allow LOCK TABLE WRITE lock a non-existent table."