Bug-6781: Inbound and outbound connection attempts from controller are not synchronized

Bug-6781: Inbound and outbound connection attempts from controller are not synchronized
- created new peer session listener registry in BGPPeerRegistry for the
  outbound connection establishment logic to get notified when new peer
  session is created or destroyed
- updated outbound connection establishment logic to attempt a connection
  only when no existing session is present
- updated unit-tests

Change-Id: Ie6afb79e290c1a3c98d82af87febd6851ad200c2
Signed-off-by: Ajay <ajayl.bro@gmail.com>
(cherry picked from commit 81050d598744b6d5fab63302f747c70ff03c366f)