Add couch_db:open_doc_revs/3

[nickva]

This is a draft PR for now. Will need to add a few tests for it before it is ready

This is the companion to open_doc_revs/4 which operates on a batch of docs at a time. As a lucky coincidence, the internal open_doc_revs_int/3 function already operates on batches, so we just expose the batched variant.

This function will be the used to optimize the _bulk_get implementation as described in issue https://github.com/apache/couchdb/issues/4183. To allow incremental node updates, let's have the backend implementation in a separate commit so it could be included in separate rolling updates releases.

Issue: https://github.com/apache/couchdb/issues/4183