[devserver] Create/update timestamp when a build is used.

This timestamp will then be consumed by clean_staged_images.py to
determine when it is safe to wipe a build from the server.

BUG=chromium-os:32912
TEST=unit

Change-Id: I0c06dd55cd915332f7aa580d846adce4972f6223
Reviewed-on: https://gerrit.chromium.org/gerrit/28627
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Ready: Alex Miller <milleral@chromium.org>
2 files changed