Bug: queueSize may show non-zero when queue is empty

[mattrpav]

Tricky scenarios to run down that can lead to queue size.

1. A race condition during failover reconnect and the second message hits the store first. The second will log: "POST COMMIT FAILED/Store COMMIT FAILED"
2. During message expiration, an IOException may throw and never retry.3.
3. A race condition that logs "Duplicate message add attempt rejected"