* Implemented periodic maintenance window.
* Moved worker creation into the core module.
* Worker processes no longer create run requests, but read queued requests from
the file system.
* Build status and logs have been moved into the StrawBoss data directory.
* An initial build status is recorded when the job is queued.
* Build status is recorded for build references as well as actual versions.
So there will be a build status for "master", for example, that is
overwritten whenever "master" is built for that step.
* RunRequests now include a timestamp.
* Added a Run object to contain both a RunRequest and the corresponding
BuildStatus for that run.
* API endpoints that talk about runs now return Run objects instead of
RunRequests.
* Moved all data layer operations into the core module so that the
"database API" only lives in one place.
