Delay deletions of stale files

[ctz]

The race condition that this avoids is as follows: a revocation check races a fetch. The revocation first read the manifest, and finds files [A, B, C]. Now the fetch obtains the new manifest, which contains [B, C]. It saves the new manifest. It determines file A is unused, so it deletes it. Now the revocation check loads files from its (in memory manifest) and finds A to be missing.

We avoid this by having the fetch process avoid deleting files that are referenced by any prior manifest.

This is another component in fixing #31

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing jbp-delay-deletions (0213745) with main (2fc62dc)}