Jupyverse install and launch strategies

[davidbrochart]

Currently, JupySpace installs Jupyverse in every environment, and launches it in a subprocess. This changes the environment, and put some requirements on it.
Maybe Jupyverse could be installed in a separate environment, and this same Jupyverse could be launched for every environment.
Also, maybe Jupyverse could run concurrently in the same event loop as JupySpace.