Summary:
This adds a new private field to ColumnTransform that settles when
the transform column is done preparing. Finalize then waits for this
promise before proceeding.
A bug was occurring when the prepare promise was still pending,
but finalize was triggered indirectly, causing finalize to throw an error
due to the transform column not yet existing.
Test Plan: Tested manually.
Reviewers: jarek, dsagal
Reviewed By: jarek, dsagal
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D3288