Support for batch metadata / meta-metadata request

[quyin]

Use cases:

1. Operations on all elements of a collection, e.g. sorting citations by year
2. Monadic exploration

A set of questions need to be answered:

1. How requests will be made, in what format
2. How results will be returned, e.g. all-in-one-batch, paging, or async through websocket

Performance will be a very important factor. Careful thoughts needed for dividing work between client and server.