Implement more orderly termination of BgpServer

Do not accept incoming sessions if the BgpServer is deleted.
This stops the WorkQueue from building up after delete of a
BgpServer is triggered. Also, send a notification and delete
the session if the BgpServer is deleted when we process the
session.

Change-Id: I56f823559e700dd6bcbeb8ebe3935697e91b47d2
Fix and improve bgp_server_test

Change-Id: I92014d563335137a242ca78cdfb40a1dda5da9db