Replace use of RapidJSON with protobuf JSON utilities

[Shillaker]

The use of RapidJSON to manually parse JSON is now no longer necessary, and instead we can use protobuf tools to convert JSON coming from a client directly into a protobuf object.

This file contains our only use of RapidJSON, so working upwards from there we should be able to completely remove the dependency. To make it a clean change, we could of course keep the existing functions but just change their implementation to use the protobuf equivalent.

Breaking changes to clients

Various clients rely on the current JSON field naming scheme (cpp client, Python client, and all the experiments), so if we can preserve this that would also reduce the impact of this change.