Need `GetMany` on NodeGetter interface

[whyrusleeping]

In order to be able to use the interfaces here in place of the DAGService from go-ipfs, we need to be able to use the batched 'GetMany' call (especially for sessions).

[Stebalien]

https://github.com/ipfs/go-ipld-format/pull/8/files#diff-82bafc6cfe9281c3c369765f732714e0R26