QPID-5973: HA cluster state may get stuck in recovering

A backup queue is considered "ready" when all messages up to the first guarded
position have either been replicated and acknowledged or dequeued.

Previously this was implemented by waiting for the replicationg subscription to
advance to the first guarded position and wating for all expected acks. However
if messages are dequeued out-of-order (which happens with transactions) there
can be a gap at the tail of the queue. The replicating subscription will not
advance past this gap because it only advances when there are messages to
consume. This resulted in backups stuck in catch-up. The recovering primary has
a time-out for backups that never re-connect, but if they connect sucessfully
and don't disconnect, the primary assumes they will become ready and waits -
causing the primary to be stuck in "recovering".

The fixes is to notify a replicating subscription if it becomes "stopped"
because there are no more messages available on the queue. This implies that
either it is at the tail OR there are no more messags until the tail. Either way
we should consider this "ready" from the point of view of HA catch-up.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1616702 13f79535-47bb-0310-9956-ffa450edef68