Add support to GC to invoke a callback with the image+storages removed. Only images whose storage was also removed will be sent to the callback. This will be used by security scanning for its own GC in the followup change.
The RepositoryActionCount table can have entries added while a repository deletion is in progress. We now perform the repository deletion under a transaction and explicitly test for RAC entries in the deletion unit test (which doesn't test interleaving, but it was missing this check).
Fixes#494
Implement the minimal changes to the local filesystem storage driver and feed them through the distributed storage driver.
Create a digest package which contains digest_tools and checksums.
Fix the tests to use the new v1 endpoint locations.
Fix repository.delete_instance to properly filter the generated queries to avoid most subquery deletes, but still generate them when not explicitly filtered.
