v0.9.1

A pi session no longer misses mail that arrives just as it finishes a reply.

After upgrading, run piggery restart: the daemon updates the pi integration it installed to 9
(omp to 9 and dsh to 10, which share its adapter file and behave as before). Then restart the pi
sessions that were open, or run /reload in them.

• Fix: mail that reached a pi session at the very end of a reply was shown to the model, and pi
  went on working with it, but piggery took the turn as finished. Mail sent after that waited,
  unannounced, until pi stopped for good or the model checked its inbox. The turn now stays open
  while pi works on that mail, so later mail is steered in as usual and nothing is acked before
  the model has read it.
• Fix: a stopped worker no longer leaves an empty directory under ~/.piggery/run/. Existing
  empty ones go when piggery gc removes their worker.