Empty channels were only cleaned up on PART, not QUIT.

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.

The rationale for why regenerateMembersCache didn't need to hold the Lock()
throughout was subtly wrong. It is true that at least some attempt to
regenerate the cache would see *all* the updates. However, it was possible for
the value of `result` generated by that attempt to lose the race for the final
assignment `channel.membersCache = result`.

The fix is to serialize the attempts to regenerate the cache, without adding
any additional locking on the underlying `Channel` fields via
`Channel.stateMutex`. This ensures that the final read from `Channel.members`
is paired with the final write to `Channel.membersCache`.

Rename getters in conformance with the "Effective Go" styleguide recommendation:
https://golang.org/doc/effective_go.html#Getters

Specifically, if you joined a channel while someone else was trying to part. the Join method would grab the lock, the Quit method would queue to grab the lock, the Join method would unlock and then try to regrab the lock, and it would get into a situation where nobody would have the lock and everyone would be waiting for it.

This caused weird oddities with clients.

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.