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_plugin.cc

@@ -21,7 +21,7 @@
 #include "sql_locale.h"
 #include "sql_plugin.h"
 #include "sql_parse.h"          // check_table_access
-#include "sql_base.h"                           // close_thread_tables
+#include "sql_base.h"                           // close_mysql_tables
 #include "key.h"                                // key_copy
 #include "sql_show.h"           // remove_status_vars, add_status_vars
 #include "strfunc.h"            // find_set
@@ -1511,8 +1511,8 @@ static void plugin_load(MEM_ROOT *tmp_root, int *argc, char **argv)
     sql_print_error(ER(ER_GET_ERRNO), my_errno);
   end_read_record(&read_record_info);
   table->m_needs_reopen= TRUE;                  // Force close to free memory
+  close_mysql_tables(new_thd);
 end:
-  close_thread_tables(new_thd);
   /* Remember that we don't have a THD */
   my_pthread_setspecific_ptr(THR_THD, 0);
   DBUG_VOID_RETURN;