Set users as being lurkers again after a certain time of inactivity.

Active (non-lurker) users are set as inactive again after
`circe-active-users-timeout' seconds.  When they once-again become
active, they emit a "user is active again" message.

Related functions/variables:
`circe-active-users-timeout'
`circe-active-users-cleanup-interval'
`circe-format-server-lurker-activity'
`circe-lurker-mark-as-active'
`circe-active-users-cleanup'
`circe-active-users-cleanup-timer'

What used to be `circe-format-server-lurker-activity' is now
`circe-format-server-lurker-first-activity'.