make downloader more robust

[jshook]

This does a couple things to make downloads more defensive:

1. Recognizes when multiple (dataset,facet) configs point to the same file and
   • for query vectors, allows it
   • for base vectors, throws an error (for now, this is not supported without windowed reads)
   • for ground truth vectors, throws an error
2. Recognizes when different instances of the same logical (dataset,facet) config points at potentially different paths.
   • This is a hard error. Before, this was a first-finder wins scenario, which is non-deterministic.
3. • For scenarios where a single file was shared, make the downloader concurrently safe.
   • Before it was possible for two different threads to use the same physical output buffer, but only with S3.

These are defensive changes which simply cause these types of errors to be surfaced. The bias here is to avoid surprises or non-deterministic results.

[github-actions]

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