Add Completeness Checking Fetcher

[Qinusty]

Initial completeness checking implementation for the fetch service.

To check:

• Should the response be NotFound on a failed completeness check?
  • Spec mentions ABORTED for failed consistency checks
• We need to store referenced blobs and check these during a fetch (maybe also at push time although I am unsure if this is required)

Addresses #4

[tomcoldrick-ct]

Should the response be NotFound on a failed completeness check?

The spec seems to tell us to use ABORTED, but I'm not sure if clients will handle this gracefully right away... I'd say it's best to cautiously use ABORTED and patch clients if we have problems?

We need to store referenced blobs and check these during a fetch

Yup, that's just a matter of updating the Asset proto to have fields for this and checking them too in the CompletenessCheckingFetcher.

(maybe also at push time although I am unsure if this is required)

If this is the case we would also want to check the original blob on push too. From a purely "KISS" perspective, I think it's best to trust clients to not push references to things which aren't in CAS, given this should be checked at Fetch time with the completeness checker.

[Qinusty]

Given recent discussions around the lack of GetTree in Buildbarn, this PR needs further thought to ensure that FetchDirectory can work efficiently with REv2/RAv1.