Every PyPI download with cooldown on read the JSON API first, cached
files included. Without cache_metadata that was a full /pypi/<name>/json
per file.
A download that looks up the JSON API now stores the release's newest
upload time on its versions row. A cached file is then checked against
the stored time without asking upstream. A file that isn't cached is
still looked up, since files can be added to a release later, and the
stored time is refreshed. A stored time inside the window withholds the
release without a lookup.
