Handling pending messages when client is stopped

[srikanthccv]

This came up during a related discussion on the 10th Jan SIG meeting. The client Stop doesn't say anything about the scheduled pending message deliverability although the current implementation discards all pending messages and returns. We should explicitly specify the expectation.

1. Drop the pending messages and return
2. Flush all the scheduled messages and send agent disconnect as the final message Set agent_disconnect on disconnect #179.