nix-store --delete doesn't work on binary caches

Currently there is no real way to manage the size of a binary cache. The functionality required to enable this use case is NIX_REMOTE=file:///path/to/binary/cache nix-store --delete <path>, which currently returns: error: requested operation is not supported by store.

Not all of us who host nix channels and binary caches have infinite storage.

To implement this effectively, support for the operation nix-store --query --referrers will also have to be implemented for binary caches, since nix-store --delete needs to fail if you attempt to delete something that is referred to by another path in the store.

It seems like this should be really easy to implement: to implement --query --referrers, search for the path reference in all of the .narinfo files, and return the ones that contain it. To implement --delete, first check that there are no referrers, and then delete the .narinfo and the .nar if there are none.

I would implement this but I suck at writing C++.