Store raw JSON, not gvariant-ized version
Store raw JSON, not gvariant-ized version

This is less efficient, but it's a lot easier for external
(e.g. golang) tools to parse without having to speak GVariant.

The downside is we re-parse JSON for each checkout, but oh well.

One can now inspect the layer JSON via e.g.

```
$ ostree --repo=repo show dockerimg/695b0cde8e9d4fb2ce79f9051f3a459aa9a2453ec2164a2eae7ac5851bb9032c --print-metadata-key docker.layer
```

cgwalters/dlayer-ostree,cgwalters/dlayer-ostree