MDEV-21953 deadlock between BACKUP STAGE BLOCK_COMMIT and parallel repl.

The issue was:
T1, a parallel slave worker thread, is waiting for another worker thread to
commit. While waiting, it has the MDL_BACKUP_COMMIT lock.
T2, working for mariabackup, is doing BACKUP STAGE BLOCK_COMMIT and blocks
all commits.
This causes a deadlock as the thread T1 is waiting for can't commit.

Fixed by moving locking of MDL_BACKUP_COMMIT from ha_commit_trans() to
commit_one_phase_2()

Other things:
- Added a new argument to ha_comit_one_phase() to signal if the
  transaction was a write transaction.
- Ensured that ha_maria::implicit_commit() is always called under
  MDL_BACKUP_COMMIT. This code is not needed in 10.5
- Ensure that MDL_Request values 'type' and 'ticket' are always
  initialized. This makes it easier to check the state of the MDL_Request.
- Moved thd->store_globals() earlier in handle_rpl_parallel_thread() as
  thd->init_for_queries() could use a MDL that could crash if store_globals
  where not called.
- Don't call ha_enable_transactions() in THD::init_for_queries() as this
  is both slow (uses MDL locks) and not needed.

sql/sql_parse.cc

@@ -1841,9 +1841,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
       */
       char *beginning_of_next_stmt= (char*) parser_state.m_lip.found_semicolon;
 
-#ifdef WITH_ARIA_STORAGE_ENGINE
-    ha_maria::implicit_commit(thd, FALSE);
-#endif
+      ha_maria_implicit_commit(thd, FALSE);
 
       /* Finalize server status flags after executing a statement. */
       thd->update_server_status();
@@ -6157,9 +6155,7 @@ finish:
       trans_commit_stmt(thd);
       thd->get_stmt_da()->set_overwrite_status(false);
     }
-#ifdef WITH_ARIA_STORAGE_ENGINE
-    ha_maria::implicit_commit(thd, FALSE);
-#endif
+    ha_maria_implicit_commit(thd, FALSE);
   }
 
   /* Free tables. Set stage 'closing tables' */