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/rpl_parallel.cc

@@ -1051,10 +1051,10 @@ handle_rpl_parallel_thread(void *arg)
   server_threads.insert(thd);
   set_current_thd(thd);
   pthread_detach_this_thread();
+  thd->store_globals();
   thd->init_for_queries();
   thd->variables.binlog_annotate_row_events= 0;
   init_thr_lock();
-  thd->store_globals();
   thd->system_thread= SYSTEM_THREAD_SLAVE_SQL;
   thd->security_ctx->skip_grants();
   thd->variables.max_allowed_packet= slave_max_allowed_packet;