QPID-4287: Poor performance when a priority queue with a ring queue policy has a large backlog

LossyQueue::checkDepth was performing an unintended linear search of its
messages when a new message was added at capacity.  Since the messages are in
priority order, only the tail message on the queue needs to be compared with the
new message to determine which of them should be dropped.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1535803 13f79535-47bb-0310-9956-ffa450edef68