bgpd: GR fixes

bgpd: GR fixes

1) When a session comes up for a peer and if the peer has not adverised
   the GR capabilities, BGP sends a request to Zebra to clear any
   stale routes that might exist from that peer.
2) When OPEN message is received from the peer, clear the previously
   advertised GR capability by the peer, if the lastest received
   OPEN message does not contain the GR capability.

Signed-off-by: NaveenThanikachalam <nthanikachal@vmware.com>