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: I06152f722561d9af789c253e716c46f3d93993b9