Summary:
With this diff, when a user opens a Grist document in a browser, they will be able to view its contents without waiting for the data engine to start up. Once the data engine starts, it will run a calculation and send any updates made. Changes to the document will be blocked until the engine is started and the initial calculation is complete.
The increase in responsiveness is useful in its own right, and also reduces the impact of an extra startup time in a candidate next-generation sandbox.
A small unrelated fix is included for `core/package.json`, to catch up with a recent change to `package.json`.
A small `./build schema` convenience is added to just rebuild the typescript schema file.
Test Plan: added test; existing tests pass - small fixes needed in some cases because of new timing
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D3036
