(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/table.py

@@ -225,6 +225,11 @@ class Table(object):
     # which are 'flattened' so source records may appear in multiple groups
     self._summary_simple = None
 
+    # For use in _num_rows. The attribute isn't strictly needed,
+    # but it makes _num_rows slightly faster, and only creating the lookup map when _num_rows
+    # is called seems to be too late, at least for unit tests.
+    self._empty_lookup_column = self._get_lookup_map(())
+
     # Add Record and RecordSet subclasses which fill in this table as the first argument
     class Record(records.Record):
       def __init__(inner_self, *args, **kwargs):  # pylint: disable=no-self-argument
@@ -237,6 +242,12 @@ class Table(object):
     self.Record = Record
     self.RecordSet = RecordSet
 
+  def _num_rows(self):
+    """
+    Similar to `len(self.lookup_records())` but faster and doesn't create dependencies.
+    """
+    return len(self._empty_lookup_column._do_fast_empty_lookup())
+
   def _rebuild_model(self, user_table):
     """
     Sets class-wide properties from a new Model class for the table (inner class within the table