Implement a websocket transport.

[jmalloc]

This issue replaces #9, #10 and #11.

It is the equivalent of the "globally-scoped" websockets described in #11:

These websockets allow a client to open a single connection that can be used for multiple RPC method calls. This will require a basic envelope format that can communicate some kind of call identifier and meta-data such as the exact operation being called, structurally similar to JSON-RPC. We might even get JSON-RPC for free by modeling its structure in protobuf files and using the JSON encoding scheme.

global-scoped means hitting the / route, client can invoke any RPC method available on the server