Make non-blocking / multithreaded mode

[Mike Farnsworth]

Currently, MetaRPC requires users to process both local and remote procedure calls within the same thread, and also call update() periodically on the RPC stream in that same thread.

While this is fine in many cases, it would be nice if a stream would spawn its own thread that handles the stream updates automatically, and allows calls to remote procedures to happen from different threads.

This of course would necessitate local procedures being created with the knowledge they may also get called from a different thread than they were registered in, but that can be clearly documented.