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We can also add this as an extension but it's simple enough I'd like to add it
directly to the Datastore.

Use-case: The blockstore now has a GetSize and bitswap uses it for packing
multiple blocks into a single message. Unfortunately, this means that bitswap is
now calling blockstore.GetSize which calls datastore.Get which tends to be more
expensive than datastore.Has.

None of our actual datastores support any other types and the cast noise gets
really annoying after a while.

This adds a PersistentDatastore interface which allows datastores
to report DiskUsage().

It implementes the interface on all wrapper types, which return
0 if the wrapped datastore does not provide this method.

Related: https://github.com/ipfs/go-ipfs/pull/4550