Since we are using keyword to represent channels on the client level, use a
plist of channels on the comm layer level instead of channel slots. This allows
us to remove `jupyter-comm-channel` and simplify the relevant code somewhat.
* jupyter-channel-ioloop.el (jupyter-zmq-channel): Require.
(jupyter-channel-ioloop-add-start-channel-event): `sync` -> `zmq`
* jupyter-comm-layer (jupyter-comm--channel): Moved from `jupyter-channels.el`.
* jupyter-kernel-manager.el: `sync` -> `zmq`
* jupyter-zmq-channel-comm.el: New file.
* jupyter-channels.el: Mostly renamed to jupyter-zmq-channel.el. The
`jupyter-channel` class was moved to `jupyter-channel.el`. All that remains are
those classes dependent on `zmq`.
* test/jupyter-test.el: `sync` -> `zmq` where appropriate.
Extract `jupyter-channel` class from `jupyter-zmq-channel.el` into its own file
