Refactor RSC protocol

The RSC protocol is pretty tightly coupled with the implementation of the driver. There are calls from the driver to the protocol and vice versa. We should ideally have these implemented as callbacks and the driver should probably never have a protocol handler reference. I like the way Avro defines the protocol in the Netty implementation, having used it in Flume. Define a bunch of interfaces that are implemented by the "handler" - in this case the driver - cleanly separates the protocol from the driver.