Host state database

Converts the host state persistence system to use boltdb instead of a json flatfile.  Fixes #532.

Changes to state of a job are now updated individually, rather than requiring complete reserialization of all jobs.
The default directory needs to be created, see the bootstrap error in [this CI log](https://s3.amazonaws.com/flynn-ci-logs/20141210171417-cf050ab1-build-a1f85afd96998317f61d8d8cbf9f499af98842f2-2014-12-10-17-16-11.txt):

```
HostID: c4fa0ba3 - cat /tmp/flynn-host.log
...
panic: could not open db: open /var/lib/flynn/host-state.bolt: no such file or directory
```