Logout left contacts seeing the user online until the me topic idle timeout
fired presUsersOfInterest(off), and even that frame was silently dropped
for p2p routes by handlePresence's xoriginal check. MarkUserOffline now
returns the remaining session count and cleanUp notifies users of interest
immediately once it hits zero; p2p presence is delivered straight to the
contact's me topic, mirroring presSingleUserOffline. Zentao #22.