Old downloads provided after deletion and replacement

[HebaruSan]

Description (What went wrong?)

If a mod author deletes a release and re-uploads a changed file with the same version number, downloading that release gives the old file for a while (exact interval uncertain).

Reproduction Steps (What did you do?)

Investigation of KSP-CKAN/NetKAN#8055 revealed:

Timestamp	Event
2020-07-21T08:01:46	Original upload
2020-07-21T08:49:53	Replacement upload
2020-07-21T09:13	Latest download by CKAN bot, still the old file! (timestamp reported by techman83)
2020-07-22T13:13	New file retrieved after manual client cache purge

• https://spacedock.info/mod/1808/OPT%20Reconfig

The CKAN bot caches its downloads, and it compares release dates to its own local timestamps and purges files if they're stale. That's why the timestamp in the cache was 24 minutes after the second upload. CKAN's already doing everything it can to keep up to date, but SpaceDock is serving stale files.

Download caching is done by Apache Traffic Server, and it has no way of knowing whether a given file is stale.

Expected Behavior (What do you think should have happened instead?)

Downloading the file after T08:49 should have given the newer file.

Extra Information (Screenshots/Error Messages/Javascript Console Output)

Related to #184.

[HebaruSan]

https://docs.trafficserver.apache.org/en/7.1.x/admin-guide/storage/index.en.html#removing-an-object-from-the-cache

[DasSkelett]

Because I have it, a nice screenshot of the bug in action:

Look at the file sizes. The frontend immediately gets the new file size, but ATS still returns the old file with a different size.