* fix a number of election issues

- the most major change here is that we now try and start an hbbft
  group as soon as we get the consensus transaction, assuming that
  we've already generated our key share.  if the group is already
  started, we simply get the old pid, and new protections keep us from
  mistakenly handing this off twice.

- in the dkg handler we no longer try to gossip sets of signatures
  early, instead waiting util at least one member gets all of them.
  they're then broadcast to help other members complete.  better safe
  than sorry.

- move handoff_consensus into a cast.  there's no particular reason
  for it to be a call, and it could lead to deadlocks.  More
  protection is added to the stages of handoff so that this is safer,
  and that we don't double-install a group.

- rework cancellation logic to be more race-resistant