Store level zero-digest fast-path

[palfrey]

Currently we have some work in stores for fast-path'ing zero-digests because we can just assume they always exist and return a default item for them. This issue covers implementing them for all stores. We can't just do this in StoreLike as sometimes we use a specific store directly and need to actually handle this properly there.

We should do this at least for all of the base stores (ones that store stuff themselves, not in a child store) as those have most of the overhead this helps, but it should be an improvement for the higher-level stores as well.

• CompletenessChecking
• Compression
• Dedup
• ExistenceCache
• FastSlow
• Filesystem Do not need to store zero-length filesystem files #2033
• GCS GCS do not upload zero #1995
• gRPC
• Memory
• Mongo
• NoOp (not needed)
• Ontap
• Redis
• Ref (not needed)
• S3
• Shard (maybe just passthrough)
• SizePartitioning (again, maybe just passthrough)
• Verify