MessageConnection onRequest and onNotification should allow removal of handler

[AArnott]

I need to support the scenario of RPC methods that are only exposed for a scoped timeframe. For example, I want to add a handler for $/invokeProxy/0/helloWorld that I can remove later (when object 0 should be garbage collected).

The onRequest and onNotification methods don't currently provide a way to revert the operation. Perhaps they can return an IDisposable?