Datastore batch get interface

[vyzo]

We currently only support a single object api through /data/get/{objectId}.
In order to facilicate object retrieval in client applications (eg aleph data expansion, indexer ingestion etc) we also need a batch get interface.

We can implement this witj a /data/get api, which reads a sequence of object keys and returns a stream of data objects as a stream.

[vyzo]

Relevant for mediachain/aleph#113