* Fix #684
* Fix #683
* Add `CHANSERV CLEAR`
* Allow mode changes from channel founders even when they aren't joined
* Operators with the chanreg capability are exempt from max-channels-per-account
* Small fixes and cleanup

* limit the total number of registrations per account
* when an account is unregistered, unregister all its channels

1. Implement persistent channel keys (#208)
2. Persist changes to registered channel modes
3. Backend changes for persisting operator status (partial #198)

This moves channel registration to an eventual consistency model,
where the in-memory datastructures (Channel and ChannelManager)
are the exclusive source of truth, and updates to them get persisted
asynchronously to the DB.

As well, add channel registration and re-applying founder privs on the first client joining the channel. I'm going to re-architect our modes system to better acocunt for this sort of change.