Deprecate protobufs?

Was wondering if people were ok with completely deleting the dependency on protobufs. It would force people to use Rethinkdb >=1.13 to use the latest revise version. Keeping the protobufs is a little more work on the short term. In the long term, the query namespace can be rewritten to optimise for the whole JSON stuff with the protobuf stuff out of the way.

The issue names no files or tests. First inspect how protobufs and the query namespace are used, then assess the impact of requiring RethinkDB >=1.13; done would mean an agreed migration or deletion plan with the resulting JSON-based query path validated.

Written by the indexing model from the issue text.