Gateway should rehydrate Sphere revisions upon receipt

[cdata]

Currently we sent sparse bundles of blocks when transmitting Sphere revisions to and from the gateway. This means that in order to retain the full IPLD representation of a sphere at a given revision, the receiver of the sparse bundle must "rehydrate" the revision upon receipt.

Currently, the gateway can receive revisions, but it does not rehydrate them. The functionality to rehydrate a sphere at a given revision is already part of the Sphere's view API. We just need to make sure we implement it in the gateway's push handler.