(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/useractions.py

@@ -8,6 +8,7 @@ import six
 from six.moves import xrange
 
 import acl
+import gencode
 from acl_formula import parse_acl_formula_json
 import actions
 import column
@@ -83,6 +84,10 @@ def is_hidden_table(table_id):
   return table_id.startswith('GristHidden_')
 
 
+def is_user_table(table_id):
+  return not (is_hidden_table(table_id) or gencode._is_special_table(table_id))
+
+
 def useraction(method):
   """
   Decorator for a method, which creates an action class with the same name and arguments.