HA memory leak in primary broker when overwriting messages in a ring queue

From https://bugzilla.redhat.com/show_bug.cgi?id=1318180

HA memory leak in primary broker when overwriting messages in a ring queue

ReplicatingSubscription accumulates IDs of dequeued messages to send on
dispatch. It should clear the accumulated IDs once sent. Due to a merge error,
since:

014f0f3 QPID-4327: HA TX transactions: basic replication.

The ID set is not cleared, causing it to accumulate memory slowly.

This leak would be particularly noticeable on a busy ring-queue since a
ring-queue generates a dequeue event for every enqueue once it reaches its max
size.