The digest-aware cache check discarded a stale entry before its caller
took the key. A slow caller could delete an entry another caller's
fetch had just committed, and everyone sharing that fetch then failed
to open it.
The check now only reports the miss. The caller running the shared
fetch discards the entry under the key, after the recheck, so one a
previous fetch refreshed is served, not deleted. The mismatch warning
fires once per refresh instead of once per request.
Swift HEAD no longer discards either, having no fetch to do it under.
It probes upstream as before, and the next GET replaces the entry.
Different digests or URLs, or no digest, use different keys and can
still collide on the storage path. That is the storage layout follow-up.
