Simplify stasis logic for people who leave the channel.

Instead of setting make_stasis as soon as they leave, then setting it
again with a negative number if they rejoin, we can just set it when
they actually reach the timer and are killed. We were actually doing
*both* up to now (which was probably what was causing the double stasis
mentioned in issue #8), so I just removed the make_stasis calls when
people leave and come back, and kept the one when they get killed.