Limit number of ReplicationDue sent per ReplicationRead

[volkerstampa]

If update-notifications are enabled and location A replicates events from location B, B sends ReplicationDue messages to A when new events are written to the log (and there is no pending replication read from A). B keeps sending these ReplicationDue messages even after A was shutdown resulting in corresponding WARN logs from akka-remote in B's application log.

To avoid spamming the application log in such circumstances the number of ReplicationDue sent to other locations on received replication read requests should be limited.