See discussion on #68.

Previously, we were applying defaults before the user had completed
registration. This meant that the number of invisible users was
incremented when the user connected, and then the total was incremented
when registration was completed.

Now both counters are updated at the same time. If a user disconnects
prior to registration, +i has not yet been applied so it would not be
decremented.

1. TAGMSG were incorrectly being considered multilines, because
   Is512() was checking the wrong thing
2. Playback of TAGMSG should depend on event-playback, not on message-tags

This ensures that counts like the number of invisible users are updated
correctly.

Construct a hash set of the user's channels and check that rather than
querying channel membership, to reduce the number of locks that need to be
acquired.