Simplify RpcContext and associated serialization

[eddyashton]

PR #501 leaves open the issue of precisely what is needed in the context. Currently it contains the raw data, the parsed JSON object, and most of the parsed fields. If possible, it would ideally only contain the parsed fields (offloading parsing work to followers), but for BFT we may need to go in the opposite direction and send only the raw data, reparsing the entire thing on the receiving node.

Once this is clear, we should ensure only the required fields are serialized between nodes, and an appropriate Context is reconstructed on the far side.

Originally posted by @eddyashton in #501

[eddyashton]

This is now done - RpcContext exposes an explicit get_serialised_request() for forwarding/PBFT replication, and this contains the entire raw request to be reparsed on the receiving node.