A fix for Bug#41158 "DROP TABLE holds LOCK_open during unlink()".

Remove acquisition of LOCK_open around file system operations,
since such operations are now protected by metadata locks.
Rework table discovery algorithm to not require LOCK_open.

No new tests added since all MDL locking operations are covered
in lock.test and mdl_sync.test, and as long as these tests
pass despite the increased concurrency, consistency must be
unaffected.

mysql-test/t/disabled.def:
  Disable NDB tests due to Bug#55799.
sql/datadict.cc:
  No longer necessary to protect ha_create_table() with
  LOCK_open. Serial execution is now ensured by metadata
  locks.
sql/ha_ndbcluster.cc:
  Do not manipulate with LOCK_open in cluster code.
sql/ha_ndbcluster_binlog.cc:
  Do not manipulate with LOCK_open in cluster code.
sql/ha_ndbcluster_binlog.h:
  Update function signature.
sql/handler.cc:
  Implement ha_check_if_table_exists().
  @todo: some engines provide ha_table_exists_in_engine()
  handlerton call, for those we perhaps shouldn't
  call ha_discover(), to be more efficient.
  Since currently it's only NDB, postpone till
  integration with NDB.
sql/handler.h:
  Declare ha_check_if_table_exists() function.
sql/mdl.cc:
  Remove an obsolete comment.
sql/sql_base.cc:
  Update to a new signature of close_cached_tables():
  from now on we always call it without LOCK_open.
  Update comments.
  Remove get_table_share_with_create(), we should
  not attempt to create a table under LOCK_open.
  Introduce get_table_share_with_discover() instead,
  which would request a back off action if the table
  exists in engine.
  Remove acquisition of LOCK_open for 
  data dictionary operations, such as check_if_table_exists().
  Do not use get_table_share_with_create/discover for views,
  where it's not needed.
  Make tdc_remove_table() optionally acquire LOCK_open
  to simplify usage of this function.
  Use the right mutex in the partitioning code when
  manipulating with thd->open_tables.
sql/sql_base.h:
  Update signatures of changes functions.
sql/sql_insert.cc:
  Do not wrap quick_rm_table() with LOCK_open acquisition, 
  this is unnecessary.
sql/sql_parse.cc:
  Update to the new calling convention of tdc_remove_table().
  Update to the new signature of close_cached_tables().
  Update comments.
sql/sql_rename.cc:
  Update to the new calling convention of tdc_remove_table().
  Remove acquisition of LOCK_open around filesystem
  operations.
sql/sql_show.cc:
  Remove get_trigger_table_impl().
  Do not acquire LOCK_open for a dirty read of the trigger
  file.
sql/sql_table.cc:
  Do not acquire LOCK_open for filesystem operations.
sql/sql_trigger.cc:
  Do not require LOCK_open for trigger file I/O.
sql/sql_truncate.cc:
  Update to the new signature of tdc_remove_table().
sql/sql_view.cc:
  Do not require LOCK_open for view I/O.
  Use tdc_remove_table() to expel view share.
  Update comments.
sql/sys_vars.cc:
  Update to the new signature of close_cached_tables().

sql/sql_trigger.cc

@@ -394,9 +394,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
   /*
     We don't want perform our operations while global read lock is held
     so we have to wait until its end and then prevent it from occurring
-    again until we are done, unless we are under lock tables. (Acquiring
-    LOCK_open is not enough because global read lock is held without holding
-    LOCK_open).
+    again until we are done, unless we are under lock tables.
   */
   if (!thd->locked_tables_mode &&
       thd->global_read_lock.wait_if_global_read_lock(thd, FALSE, TRUE))
@@ -516,11 +514,9 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
       goto end;
   }
 
-  mysql_mutex_lock(&LOCK_open);
   result= (create ?
            table->triggers->create_trigger(thd, tables, &stmt_query):
            table->triggers->drop_trigger(thd, tables, &stmt_query));
-  mysql_mutex_unlock(&LOCK_open);
 
   if (result)
     goto end;
@@ -1680,9 +1676,6 @@ bool add_table_for_trigger(THD *thd,
   @param db       schema for table
   @param name     name for table
 
-  @note
-    The calling thread should hold the LOCK_open mutex;
-
   @retval
     False   success
   @retval
@@ -1912,14 +1905,10 @@ bool Table_triggers_list::change_table_name(THD *thd, const char *db,
 
   /*
     This method interfaces the mysql server code protected by
-    either LOCK_open mutex or with an exclusive metadata lock.
-    In the future, only an exclusive metadata lock will be enough.
+    an exclusive metadata lock.
   */
-#ifndef DBUG_OFF
-  if (thd->mdl_context.is_lock_owner(MDL_key::TABLE, db, old_table,
-                                     MDL_EXCLUSIVE))
-    mysql_mutex_assert_owner(&LOCK_open);
-#endif
+  DBUG_ASSERT(thd->mdl_context.is_lock_owner(MDL_key::TABLE, db, old_table,
+                                             MDL_EXCLUSIVE));
 
   DBUG_ASSERT(my_strcasecmp(table_alias_charset, db, new_db) ||
               my_strcasecmp(table_alias_charset, old_table, new_table));