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This will allow many consumers of `DAGService` to take `NodeGetter` instead and
implementing `GetMany` for all `NodeGetter`s is pretty trivial.

We're going to want to do this eventually and we have to refactor anyways so we
might as well do this now.

Fixes ipfs/go-ipfs#4010

We'll just live with this name.

This way, not *all* DAGServices need to implement this interface, they can just
implement it as an optimization.

This is a WIP/RFC attempt at extracting DAGService from go-ipfs.