Host checkin freshness

This will handle [this issue](https://github.com/PeopleAdmin/qtrix/issues/10).

As hosts ask for queue lists, we track the request as a ping with a HostManager.  If we detect that any hosts have not checked in for a time (15 seconds by default), we blow away the matrix and override claims so that the allocation of queues to hosts will be regenerated from scratch as they all check in next time.  This has the following effects:

* We handle hosts going offline as far as the matrix is concerned, qtrix will balance elastically as hosts come online/go offline.
* Overrides that were being served by a particular host will be picked up and served by another host if the first goes offline.

This is a gating issue for hitting the [public release milestone](https://github.com/PeopleAdmin/qtrix/issues?milestone=1&state=open)

- [x] Test branch with PA workers and ensure that rebalancing happens correctly under stressque load.