(core) Log number of rows in user tables in data engine

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

sandbox/grist/engine.py

@@ -1175,6 +1175,13 @@ class Engine(object):
     """
     self._unused_lookups.add(lookup_map_column)
 
+  def count_rows(self):
+    return sum(
+      table._num_rows()
+      for table_id, table in six.iteritems(self.tables)
+      if useractions.is_user_table(table_id)
+    )
+
   def apply_user_actions(self, user_actions, user=None):
     """
     Applies the list of user_actions. Returns an ActionGroup.