Track recently-parted non-lurker users and show their JOIN.

Upon parting/quitting, an active (non-lurker) user is added to
`circe-recently-parted-active-users', from which they are removed
after `circe-parted-users-timeout' seconds.  If a user re-joins before
the time-out, they emit a re-join message.

Related functions/variables:
`circe-parted-users-timeout'
`circe-format-server-user-rejoin'
`circe-channel-add-user'
`circe-channel-remove-user'
`circe-channel-parted-users-cleanup'
`circe-channel-parted-active-users'
`circe-lurker-mark-as-active'