circe-channel-killed handling

[defanor]

When leaving a channel by killing its corresponding buffer, there's a
chance to get that buffer reopened if a new message arrives before we
actually leave that channel.

It can be handled in a few ways, including the way it works now, but
now it looks like a bug. If it's intentional, then
circe-channel-killed shouldn't be invoked once again when killing
the second buffer.

[jorgenschaefer]

Hm. Yeah, unfortunate race condition, that. If someone wants to work on a "fix" of some sort, feel free.