Tweak BgpPeerNotification log to display TCP endpoint if there's no Peer.

Add tests to exercise the code that sends a Notification when BgpPeer is
marked deleted or is being closed.
Add test for deferred peer close.
Merge pull request #626 from Juniper/nsheth_notification_log

Tweak BgpPeerNotification log to display TCP endpoint if there's no Peer...