Notebook State that should be maintained server-side

[Zsailer]

Using this issue to list all areas where we should intercept messages from the kernel and write their data directly into the server-side YNotebook.

• 1. Execution State
• 2. Cell execution time.
• 3. Currently executing cell
• 4. Pending cells
• 5. Notebook Language Info.
• 6. Cell Outputs

[Zsailer]

In #18, I'm running into an issue where I need the language info to come from the ydoc.

[ellisonbg]

I like this idea - no reason that frontend should need to calculate all of those things.

[Zsailer]

1, 3, and 4 were fixed in #146

[Zsailer]

6 was handled by #30