Cell execution order

[Wh1isper]

Problem

If the front-end runs multiple cells at the same time,seems no way to determine the order of them on server side. I'm not sure if this is a bug or feature, but it's not really consistent with the current websocket experience.

Proposed Solution

Maybe we should allow the api to receive arrays of cells to ease this?

In the meantime, we may need an execution queue to execute each set of cells sent from the frontend.

Additional context