2.4.0

[Postfix] Abnormal Exit of Xtrabackup caused replication stop

Summary
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To obtain a consistent index before locking, XPAXOS replication
will be stopped. After the backup ends, XPAXOS replication will be
restarted. If Xtrabackup exits abnormally, it may cause the xpaxos
replication status to remain in a stopped state.

Fix
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Force use FTWRL when bakcup XCluster instance to get a valid index.