Add mechanism to access JsonRpcRequest instance from service implementation

It would be useful to be able to access the JsonRpcRequest (or in my particular case the RequestId) from service implementations so that I can do out-of-band association between client and server requests. Specifically, I'm trying to post-facto recreate a session after it's been terminated with information stored on both parties. To avoid async and threading related race conditions I think it would need to be provided as a special argument to the handler function, but I don't know what the performance implications of adding this would be or the implementation approach.

Start by tracing how service implementations are invoked and how JsonRpcRequest data reaches the handler function. Investigate whether a special handler argument can expose the request or RequestId without introducing async or threading races, and assess the performance implications. Done requires an agreed API and implementation approach, but the issue does not identify files or tests.