MONITOR support for UserTrack
MONITOR should be used where available to track users, instead of an expiration timer. This will be useful for bots, but probably not clients (as clients will want MONITOR reserved for the user).
Whilst we're at it, since we have outgoing hooks, ISON support may be useful to implement at the same time, despite its deficiencies. In PyIRC 2 we used a recurring timer for that.