(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/test_lookups.py

@@ -399,7 +399,9 @@ return ",".join(str(r.id) for r in Students.lookupRecords(firstName=fn, lastName
     self.assertPartialOutActions(out_actions, {
       "calls": {
         # No calculations of anything Schools because nothing depends on the incomplete value.
-        "Students": {"#lookup#firstName:lastName": 1, "schoolIds": 1, "schoolCities": 1}
+        "Students": {
+          "#lookup#firstName:lastName": 1, "schoolIds": 1, "schoolCities": 1, "#lookup#": 1
+        }
       },
       "retValues": [7],
     })
@@ -408,7 +410,9 @@ return ",".join(str(r.id) for r in Students.lookupRecords(firstName=fn, lastName
     out_actions = self.add_record("Students", firstName="Chuck", lastName="Norris")
     self.assertPartialOutActions(out_actions, {
       "calls": {
-        "Students": {"#lookup#firstName:lastName": 1, "schoolIds": 1, "schoolCities": 1},
+        "Students": {
+          "#lookup#firstName:lastName": 1, "schoolIds": 1, "schoolCities": 1, "#lookup#": 1
+        },
         "Schools": {"bestStudentId": 1}
       },
       "retValues": [8],