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_class.cc

@@ -1383,7 +1383,11 @@ void THD::update_all_stats()
 void THD::init_for_queries()
 {
   set_time(); 
-  ha_enable_transaction(this,TRUE);
+  /*
+    We don't need to call ha_enable_transaction() as we can't have
+    any active transactions that has to be commited
+  */
+  transaction.on= TRUE;
 
   reset_root_defaults(mem_root, variables.query_alloc_block_size,
                       variables.query_prealloc_size);
@@ -7309,7 +7313,6 @@ wait_for_commit::~wait_for_commit()
   mysql_cond_destroy(&COND_wait_commit);
 }
 
-
 void
 wait_for_commit::wakeup(int wakeup_error)
 {