PCEP session remain registered when a PCEP CLOSE message is received

The PCEP CLOSE message was not handled correctly by ODL. Right now when a PCEP CLOSE message was received by the controller, it simply disconnect the PCEP peer without unregistering it from PCEP topology. Thus the closed PCEP session will stay in the topology and user can still see it in jolokia after the session is gone. As follow-up problem, when the PCEP session comes back again, it fails to register on PCEP topology again as previous registration was not removed, so ODL will not actively sends out keep-alive messages to remote peer, then the DEAD_TIMER problem was triggered.

This bug can only be triggered when the router sends out PCEP CLOSE message before it drops the PCEP session. However, route doesn't always do it, most of the time it just drop the TCP connection, and in this case the bug will not be triggered.