Add support for MessagePack, or other compact/binary format

JSON is great for interoperability, but not so much for efficient network communication.

I would love to see a future version of CouchDB that supports a more compact data streaming format, such as MessagePack. (Ideally in both CouchDB and PouchDB, as PouchDB is really where it would be the most useful--but it needs CouchDB support, first).

I haven't looked deeply into it, but I expect MessagePack would be a good choice, as can fairly cleanly be converted 1:1 to/from JSON.

## Desired Behaviour

This should not, in any way, replace JSON as the primary content type used by CouchDB. I would see it becoming an optional `Accept:` and `Content-Type:` header. The conversion should be trivially handled in a middleware, so it shouldn't need to affect any core logic, and would make for an easy prototype implementation entirely outside of CouchDB.

So for incoming requests (POSTs/PUTs, etc):

If the Content-Type header matches MessagePack, simply convert to JSON, then pass along to normal processing.

If the Accept: header matches MessagePack, perform the normal operation, then before streaming the response, convert from JSON to MessagePack.

## Additional context

- There may be some corner cases around data conversion that need addressing to ensure 100% compatibility, particularly around [large numbers](https://github.com/tinylib/msgp/wiki/The-MessagePack-Type-System#base-types).

- There's [some confusion](https://github.com/msgpack/msgpack/issues/194) about which MIME type to use for MessagePack. If this proposal and format are ultimately selected, a small amount of bike-shedding will need to be done.

- There may be other data formats to consider, as well. I'd love to hear other suggestions. (Although, IMO, it should be an existing, open standard, rather than inventing our own.)

- BSON might be a candidate, but is not as attractive, at first blush, as it's both larger than MessagePack, and it's not as easily converted 1:1 to/from JSON.

- As a first step, writing a stand-alone CouchDB proxy and a PouchDB plugin would probably be worth-while, to allow testing the functionality for compatibility and performance. If there is broader interest, I may try to work on this in time.

The proposal concerns optional Accept and Content-Type handling for POSTs, PUTs, and streamed responses, with CouchDB support first and a possible PouchDB plugin or proxy. The format choice, MIME type, conversion edge cases, implementation location, and completion criteria remain undecided.

Written by the indexing model from the issue text.