Add CompletenessChecking for Fetch/Push services

[tomcoldrick-ct]

Currently we naïvely assume that the blobs referenced in requests/responses exist in CAS without checking. We should add something analogous to the CompletenessCheckingBlobAccess to ensure that blobs we reference actually exist.

One thing to be careful of while implementing this is that PushBlob and PushDirectory requests contain an additional pair of digest lists which we will also need to ensure exist. Given this, we'll need to add an extra field (or two, if we want to distinguish between Blob/Directory) to the Asset proto to store these digests so we can check at push and fetch time.