Add ability to shutdown the buildserver.

New requests will be rejected, but existing builds will run to
completion. Also updated the healthcheck function to only reply with
success if a shutdown is not pending. So any load balancer will know
to no longer send jobs at the server while it is shutting down.

Change-Id: I4a2224df99170deef08a780ab7328ebcebbb068b
Bugfix to GCS Retry Code

Make sure we break out of the retry loop when we succeed in finally
reading an object out.

Change-Id: I10e47c6bc3c81cd03880658191e01f3e149dfaa5