Eliminated the Kernel and Kernelspec handlers. The Websocket (ZMQ)
channels handler still remains. This required turning a few methods
into coroutines in the Notebook server.
Renamed the Gateway config object to GatewayClient in case we want
to extend NB server (probably jupyter_server at that point) with
Gateway server functionality - so an NB server could be a Gateway
client or a server depending on launch settings.
Add code to _replace_ the channels handler rather than rely on position
within the handlers lists.
Updated mock-gateway to return the appropriate form of results.
Updated the session manager tests to use a sync ioloop to call the
now async manager methods.
Kevin Bates
acba19033b