Store every supplied checksum in the reporting cluster

[alexwlchan]

Now the storage service knows about multiple manifests (#900), it should be possible to index every checksum in every manifest in the reporting cluster.

Currently we'll only store the "best" checksum, but indexing every supplied checksum would be useful. It would allow us to search on any user-supplied checksum.

This would have been particularly useful when decommissioning our old storage repository – we had a collection of SHA-1 checksums that it had created, and being able to include them in the payload manifests and search for them would have let us track what had/hadn't been migrated.