Fix incorrect socket closure in Overlay peers:
On Application exit, Overlay was calling PeerImp::close for each peer.
The implementation of PeerImp::close only canceled all pending I/O and did not
call functions necessary for proper transition of Peer state during socket
closure. The correct transition is ensured by calling PeerImp::detach. This
changes PeerImp::close to call PeerImp::detach instead, ensuring that Overlay
invariants are maintained. Specifically, that reference counts for pending I/O
on peers will be correctly unwound by canceling operations and that the Peer
object will be destroyed, thus allowing the Overlay to stop correctly.