devserver deprecation: timestamp dirs to allow cleaning up

BUG=chromium:1122385
TEST=Tested on chromeos2-devservertest.
 1. Patch the server code.
 2. Ran `curl http://localhost:8888/setup_telemetry?archive_url=gs://chromeos-image-archive/fizz-release/R84-13099.74.0`
 3. Verified a timestamp file was created with correct time.

Change-Id: Iaf8f97b8b2794a9eab739d2bc017aff4b2e84630
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2380592
Commit-Queue: Congbin Guo <guocb@chromium.org>
Commit-Queue: Sanika Kulkarni <sanikak@chromium.org>
Tested-by: Congbin Guo <guocb@chromium.org>
Auto-Submit: Congbin Guo <guocb@chromium.org>
Reviewed-by: Sanika Kulkarni <sanikak@chromium.org>
1 file changed