Refactor the BGP code in the SDN-IP application:
 * Resolves ONOS-476
 * Moved the BGP message-specific processing from class BgpSession
   to per-message type classes: BgpKeepalive, BgpNotification,
   BgpOpen, BgpUpdate
 * Minor modifications in some of the methods or BGP-specific API
   to accomodate the above change.
   No functional changes.

Change-Id: I95df128fa31c60397a279aaca25a487b7991a6e1