Transaction replay may deadlock with switchover

This happens if mariadbmon does a switchover while readwritesplit is replaying a transaction on the server being demoted. As the switchover removes the Master label from the demotion target before putting it into read-only mode, a deadlock occurs where the monitor waits for the SET GLOBAL read_only=1 to complete which is blocked by the earlier transaction that readwritesplit has open. The transaction in readwriteplit in turn is waiting for a server with a Master label to appear and thus a deadlock is formed that'll be resolved once the transaction replay times out.