circe-command-WL: Print the list of missing members as a single line.

The old implementation spammed the buffer. The new doesn't. Muuuch
better.
circe-lurker-p: Rewrite to be explicit about what it does.

At least I can now follow the logic more easily.
circe-duration-string: Round DURATION, for integer-division to work.

Also remove superfluous comment.