Regard users as being lurkers again after a time of inactivity.

Active (non-lurker) users are regarded as being 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-format-server-lurker-activity'
`circe-lurker-mark-as-active'

What used to be `circe-format-server-lurker-activity' is now
`circe-format-server-lurker-first-activity'.
Properly handle initial NAMES list, re. lurker-spam-reduction.

Upon channel join, give all users the `initial-name' property.  These
don't emit a "first activity" message if they act within
`circe-active-users-timeout' seconds after joining; afterwards they
do, with a distinct message.

Added `circe-channel-join-time' because it was needed for this.

Related functions/variables:
`circe-format-server-user-first-activity'
`circe-channel-users-synchronize'
`circe-lurker-mark-as-active'