Calculate checksum when installing image layers

##### Compute TarSum of image layer data when installing
- Adds a new `Checksum` field to the current image format to store the result of the tarsum of the file system changes.

##### Remove `jsonData` argument from `image.StoreImage`
- The argument specified the json data to save to disk when registering a new image into the image graph. If it is nil, then the given image is serialized to json and that is written by default. This default behavior is sufficient because the given image was originally deserialzed
from this jsonData to begin with.

##### Remove unused "image_set" job
- A quick grep of the code base shows that the "image_set" job registered by the TagStore is not used.

work towards #8093