gitserver: reclone janitor job runs independently of janitor period

Created by: keegancsmith

We increased how often the janitor job runs a few months ago. This inadvertedly made our reclone logic essentially always run after 45 days. Previously when we only ran once a day the jitter on reclone would non-uniformly spread the reclone time out over 45->57 days. This commit changes us from using a random jitter, to a deterministic jitter based on the repository name. This will prevent all repos recloning at the same time at a large installation.

Fixes https://github.com/sourcegraph/sourcegraph/issues/8259