Drop all traffic both ways if there are any users whose join state to the channel is unknown

[kegsay]

Requires #448.

During the period where there are unaccounted users (on startup, when connections drop) mark the room as "desynced" or equivalent. Desynced rooms should drop all traffic both ways until all users are accounted for, whereby it can go back to "synced".

This makes the matrix side UX worse because it means any intermittent connection issues will now result in dropped traffic for all users even if everyone else's connection is fine. This has been formally requested by Freenode.