UI delay caused by JsonRpc.Dispose()

[AArnott]

This bug is unique to v2.2. It was introduced in #364 in v2.2 and does not appear in v2.3 because a more complete fix was made there that kept things asynchronous throughout.

JsonRpc.Dispose() calls MessageHandlerBase.Dispose() which blocks the calling thread while waiting for reading/writing to complete. In Visual Studio this is often the UI thread or may be many threadpool threads leading to UI delays for threadpool exhaustion reasons.