Cluster-aware clients / API

@janl:
> This might end up being not a good idea, but I’d like some experimentation here. Say you’d have a CouchDB client that could be hooked into the cluster topology so it’d know which nodes to query for which data, then we can save a proxy-hop, and build clients that have lower-latency access to CouchDB. Again, this is something that Couchbase does and I think is worth exploring.

@davisp:
>Lastly, for the smart cluster clients, the work I did for COUCHDB-2791 already implements a bit of that. There's definitely more to add here to flesh it out but the surprisingly simple implementation makes me think that it'd fit in quite nicely with the HTTP refactoring work from above. I'm currently working on improving our API tests in Erlang so that I can eventually turn those branches into PRs.

Start by reviewing COUCHDB-2791 and the HTTP refactoring work mentioned in the discussion, then inspect the Erlang API tests that were being improved. The scope is exploratory, so first establish which cluster-topology and data-routing behavior the client should support. Done would require an agreed design and API test coverage for the supported behavior.

Written by the indexing model from the issue text.