Minor fixes in the mstate presence watching logic

[Gustavo Niemeyer]

These comments were made in a review, but the branch ended up moving on without them. We should come back to it soon:

- We need to observe the Dying of the presence watcher, whenever added channels are used. The Dying method doesn't exist yet.

- We should guarantee that Alive always returns true after a false, and that's not true right now because alive may have to return when facing a dead presence.Watcher. We need to return an additional error result with Watcher's Alive method so that one can tell if the false result is real or not.