Summary:
Adds a method Table._num_rows using an empty lookup map column.
Adds a method Engine.count_rows which adds them all up.
Returns the count after applying user actions to be logged by ActiveDoc.
Test Plan: Added a unit test in Python. Tested log message manually.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3275