Better async implementation

[osolmaz]

Feedback from Eden:

Oddly you're not using async where it makes sense - in the network requests - but I'm guessing you're forced into it for Jupyter. It's not essential unless you ever find yourself in a place with multiple user requests for code gen.

What would be interesting, and would use that async appropriately, is sending the request to multiple services and letting the user pick the most appropriate one.

IPythonKernel.do_execute() is currently overloaded in a hacky way and it poses problems e.g. when KeyboardInterrupt is called. Async handling needs to be overall improved.