- Sort the leases by expiration date in TunnelPool.locked_buildNewLeaseSet()
        to make later LeaseSet comparisons reliable. This cleans up the code too.
      - Fix broken old vs. new LeaseSet comparison
        in ClientConnectionRunner.requestLeaseSet(),
        so that we only sign and publish a new LeaseSet when it's really new.
        Should reduce outbound overhead both in LeaseSet publishing and LeaseSet bundling,
        and floodfill router load, since locked_buildNewLeaseSet() generates
        the same LeaseSet as before quite frequently, often just seconds apart.