Explore transferring content in chunks for incremental loading

[goanpeca]

With the work @echarles started on Virtual Notebook (lazy loading on cells on open) for JLab, having to deal with very large files, notebooks for this example opens the opportunity of trying this to the virtual notebook so only part of the notebook is transferred in chunks (Light metadata, then cells, heavy metadata, then outputs)

Related links

• 
https://github.com/ipython/ipython/wiki/IPEP-27:-Contents-Service

• 
https://en.wikipedia.org/wiki/JSON_streaming

• 
https://github.com/jupyter/jupyter_server/blob/master/jupyter_server/services/api/api.yaml

• https://github.com/jupyter/jupyter_server/blob/097676b64cb9043fd65c19d15d4df2fd9d310d50/jupyter_server/services/contents/largefilemanager.py#L12