Verify that replica nodes received checkpoint LSN on shutdown (#2939)

In case if archiving is enabled the `Postgresql.latest_checkpoint_location()` method returns LSN of the prev (SWITCH) record, which points to the beginning of the WAL file. It is done in order to make it possible to safely promote replica which recovers WAL files from the archive and wasn't streaming when the primary was stopped (primary doesn't archive this WAL file).

But, in certain cases using the LSN pointing to SWITCH record was causing unnecessary pg_rewind, if replica didn't managed to replay shutdown checkpoint record before it was promoted.

In order to mitigate the problem we need to check that replica received/replayed exactly the shutdown checkpoint LSN. But, at the same time we will still write LSN of the SWITCH record to the `/status` key when releasing the leader lock.

tests/test_ha.py

@@ -1532,7 +1532,7 @@ class TestHa(PostgresInit):
         self.ha.is_leader = true
 
         def stop(*args, **kwargs):
-            kwargs['on_shutdown'](123)
+            kwargs['on_shutdown'](123, 120)
 
         self.p.stop = stop
         self.ha.shutdown()

tests/test_postgresql.py

@@ -237,7 +237,10 @@ class TestPostgresql(BaseTestPostgresql):
     @patch.object(Postgresql, 'latest_checkpoint_location', Mock(return_value='7'))
     def test__do_stop(self):
         mock_callback = Mock()
-        with patch.object(Postgresql, 'controldata', Mock(return_value={'Database cluster state': 'shut down'})):
+        with patch.object(Postgresql, 'controldata',
+                          Mock(return_value={'Database cluster state': 'shut down',
+                                             "Latest checkpoint's TimeLineID": '1',
+                                             'Latest checkpoint location': '1/1'})):
             self.assertTrue(self.p.stop(on_shutdown=mock_callback, stop_timeout=3))
             mock_callback.assert_called()
         with patch.object(Postgresql, 'controldata',