Change event scheduling

* Changes event scheduling to only use one-shot timers instead
  of automatically repeating ones. Event scheduling will be improved
  in further patches to allow for notifications during polls like
  'Poll ends soon' as we had with the old approach. The main purpose
  of this patch is to fix the anoying problems with finishing events.
Merge branch 'foomodule-poll-scheduler' of github:rku/xmpp-crowd