[FIX] ensure activities are not executed after a user leave

It was possible that an activity could be executed after a user left the
session AND the listener was already notified. Depending on the logic
that may use the userJoined / userLeft callbacks this can lead to
unknown errors.

Change-Id: Ic1309c645f760a8b672c8282e198ec3773802167
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1210
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>