v2.1.3

Fixed

• Two replicas landing the same retried change could both apply it, leaving it in history
  twice. A change rebased onto a tip that already holds all of its changes is now reported
  as landed there instead of becoming an empty commit (as git rebase drops commits that
  become empty).

Added

• bench/chaos.sh: three replicas on Postgres behind a connection-cutting proxy, killed
  and restarted while clients land; checks the landing invariants through the public API.
  Runs nightly with the simulation.
• Documentation: how Pando is tested.