Handler to respond to GET/HEAD requests for recently uploaded NAR files.
Nix probes at {filehash}.nar[.compression_suffix] to determine whether a NAR
has already been uploaded, by responding to (some of) these requests we
avoid it unnecessarily uploading.
We don’t keep a full K/V from NAR hash to root note around, only the
in-memory cache used to connect to the castore node when processing a PUT
for the NARInfo.