* rtc: record a room's departure when the participant leaves it
RemoveParticipant removed the participant from the room, closed it, and
only then recorded leftAt. CloseIfEmpty runs every second, so a tick
landing during the close saw an empty room with a stale or zero
departure time and closed it at once: with no departure recorded it
compares against the room's creation time and empty_timeout, and with an
earlier departure it measures the grace period from that older
timestamp. Either way departure_timeout is skipped.
Record the departure where the participant is removed, under the lock
that already covers it.
* rtc: drop the removal wait from the departure timeout test
The close signal already pins the interleaving, and the room's close
state is decided by the CloseIfEmpty call itself.
Martin Njalale
5cd0f37342