pkg/store: use blobstore and metadatastore

[flokli]

This further disentangles Metadata and Blob storage.

We also stop storing (redundant) .narinfo files on disk, but breaks the data into two types, NarMeta and PathInfo.

This brings a bunch of basic consistency sanity checks.

For example, a .narinfo file can now only uploaded if it refers to a .nar file that already exists with that narhash. NarSize also needs to be correct.

In the future, we could do some reference detection on uploaded .nar files, and validate this matches with what (each) Narfile specifies.

The new interface allows easy addition of new methods, which can be used to ask GC-related questions.

The metadatastore.FileStore simply serializes all structs as json to disk, but we can now also store this in a database without a lot of effort.

Related: #9

[flokli]

I want to add some more tests before merging this, mostly integrity-related:

• ensuring you can only upload narinfo files if the narfile is there
• ensuring you can only upload a .narinfo with references if those references are in nix-casync too

I guess adding a NAR reference scanner, and validating it with what Nix sends in the uploaded .narinfo files can be moved to a separate PR.

[flokli]

I added some more tests to ensure the "foreign key constraints" are met, too.

[flokli]

This also fixes #23.