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/event_db_repository.h

@@ -91,7 +91,7 @@ public:
   bool
   load_named_event(THD *thd, LEX_STRING dbname, LEX_STRING name, Event_basic *et);
 
-  bool
+  static bool
   open_event_table(THD *thd, enum thr_lock_type lock_type, TABLE **table);
 
   bool
@@ -109,9 +109,6 @@ public:
   static bool
   check_system_tables(THD *thd);
 private:
-  void
-  drop_events_by_field(THD *thd, enum enum_events_table_field field,
-                       LEX_STRING field_value);
   bool
   index_read_for_db_for_i_s(THD *thd, TABLE *schema_table, TABLE *event_table,
                             const char *db);