Duplicate cleanup and connected-peer-counter decrement during BGP session cleanup

While trying to repro issue in https://bugs.opendaylight.org/show_bug.cgi?id=6373 (update message with missing BGP mandatory attribute received), noticed that BGP session is getting closed twice and also connected-peer-counter may get decremented twice resulting in negative counter value

2016-08-09 01:15:25,447 | INFO | entLoopGroup-5-7 | BGPSessionImpl | 185 - org.opendaylight.bgpcep.bgp-rib-impl - 0.6.0.SNAPSHOT | Closing session: BGPSessionImpl

{channel=[id: 0x58e9ec32, L:/10.29.12.199:1790 - R:/10.29.12.108:17900], state=UP}

2016-08-09 01:15:25,447 | INFO | entLoopGroup-5-7 | BGPPeer | 185 - org.opendaylight.bgpcep.bgp-rib-impl - 0.6.0.SNAPSHOT | Session with peer 10.29.12.108 terminated: BGPTerminationReason

{error=WELL_KNOWN_ATTR_MISSING}

2016-08-09 01:15:25,452 | INFO | entLoopGroup-5-7 | BGPSessionImpl | 185 - org.opendaylight.bgpcep.bgp-rib-impl - 0.6.0.SNAPSHOT | Closing session: BGPSessionImpl

{channel=[id: 0x58e9ec32, L:/10.29.12.199:1790 ! R:/10.29.12.108:17900], state=IDLE}