Use the .ls files to improve performance

Nix clients who upload with `write-nar-listing = true` also upload a `.ls` file that contains all the archive metadata.

```
nix run nixpkgs.{brotli,curl, bash} -c \
curl -v https://cache.nixos.org/$(readlink $(which bash) | cut -d/ -f4 | cut -d- -f1).ls | brotli -d
```

This could be used to quickly determine whenever a file exists and also create directory listings.