MDEV-30423 Deadlock on Replica during BACKUP STAGE BLOCK_COMMIT on XA transactions

The user XA commit execution branch was caught not have been covered
with MDEV-21953 fixes.

The XA involved deadlock is resolved now to apply the former fixes
pattern.
Along the fixes the following changes have been implemented.
- MDL lock attribute correction
- dissociation of the externally completed XA from the current
  thread's xid_state in the error branches
- cleanup_context() preseves the prepared XA
- wait_for_prior_commit() is relocated to satisfy both
  the binlog ON (log-slave-updates and skip-log-bin)
  and OFF slave execution branches.

mysql-test/suite/rpl/t/parallel_backup_xa.inc

@@ -0,0 +1,79 @@
+# Invoked from parallel_backup.test
+# Parameters:
+# $complete        = COMMIT or ROLLBACK
+# $slave_ooo_error = 1 means slave group commit did not succeed
+#
+--let $kind = Normal
+if ($slave_ooo_error)
+{
+  --let $kind = Errored out
+}
+--echo #
+--echo # $kind XA $complete
+
+--connection slave
+--source include/stop_slave.inc
+
+--connection master
+# val_0 is the first value to insert on master in prepared xa
+# val_1 is the next one to insert which is the value to block on slave
+--let $val_0 = `SELECT max(a)+1 FROM t1`
+--let $val_1 = $val_0
+--inc $val_1
+
+--connection aux_slave
+BEGIN;
+--eval INSERT INTO t1 VALUES ($val_1)
+
+--connection master
+XA START '1';
+--eval INSERT INTO  t1 VALUES ($val_0)
+XA END '1';
+XA PREPARE '1';
+--connection master1
+--eval INSERT INTO t1 VALUES ($val_1)
+--connection master
+--eval XA $complete '1'
+--source include/save_master_gtid.inc
+
+--connection slave
+if ($slave_ooo_error)
+{
+  SET @sav_innodb_lock_wait_timeout  = @@global.innodb_lock_wait_timeout;
+  SET @sav_slave_transaction_retries = @@global.slave_transaction_retries;
+  SET @@global.innodb_lock_wait_timeout =1;
+  SET @@global.slave_transaction_retries=0;
+}
+--source include/start_slave.inc
+--connection aux_slave
+--let $wait_condition= SELECT COUNT(*) = 1 FROM information_schema.processlist WHERE state = "Waiting for prior transaction to commit"
+--source include/wait_condition.inc
+--echo # Xid '1' must be in the output:
+XA RECOVER;
+--connection backup_slave
+  BACKUP STAGE START;
+--send BACKUP STAGE BLOCK_COMMIT
+--connection aux_slave
+  --sleep 1
+  if ($slave_ooo_error)
+  {
+    --let $wait_condition= SELECT COUNT(*) = 0 FROM information_schema.processlist WHERE state = "Waiting for prior transaction to commit"
+    --source include/wait_condition.inc
+  }
+  ROLLBACK;
+--let $wait_condition= SELECT COUNT(*) = 1 FROM information_schema.processlist WHERE state = "Waiting for backup lock"
+--source include/wait_condition.inc
+--connection backup_slave
+  --reap
+  BACKUP STAGE END;
+--connection slave
+if (!$slave_ooo_error)
+{
+  --source include/sync_with_master_gtid.inc
+}  
+--source include/stop_slave.inc
+if ($slave_ooo_error)
+{
+  SET @@global.innodb_lock_wait_timeout = @sav_innodb_lock_wait_timeout;
+  SET @@global.slave_transaction_retries= @sav_slave_transaction_retries;
+}