Add separate fields for column family and qualifier in REST message formats

The current format uses the archaic column field, which requires extra processing and copying to encode/decode the CF and CQ at both the server and client side.

We need to:

• Add a version field to the requests, to be enabled by clients that support the new format
• Add the new fields to the JSON, XML and protobuf formats, and logic to use them.

This should be doable in a backwards-compatible manner, with the server falling back to the old format if it receives an unversioned request.