Provide a generic way of checking whether a database is the active writer

[hkdsun]

Currently, we rely on the @@read_only variable to check whether our connection to the database is indeed to an active writer or if it's actually a read replica. This is done specifically before we use the WaitUntilReplicaIsCaughtUpToMaster struct to enter the cutover phase.

Perhaps not everybody's failover strategy is compatible with this behaviour. We could provide a configuration option on the WaitUntilReplicaIsCaughtUpToMaster struct so that the user can provide us with a query that checks this condition.