Pass document id & rev rather than the whole document

This uses the ddoc_cache support for revisions. It should reduce bandwidth overhead of copying documents around. These changes are rather minor and simply just wire in the changes provided by https://github.com/cloudant/ddoc_cache/pull/3.

I'll note that the `fabric_util:doc_id_and_rev` feels a little ad-hoc for a lightweight inlined function but it's much nicer to leave the record matching noise out of the two places its used. We still pull it apart before calling ddoc_cache so it may be a reason to support using two parameters instead but the arguments list on these rpc handlers are already growing numerous.