Take advantage of pg_stat_wal_recevier (#1513)

So far Patroni was parsing `recovery.conf` or querying `pg_settings` in order to get the current values of recovery parameters. On PostgreSQL earlier than 12 it could easily happen that the value of `primary_conninfo` in the `recovery.conf` has nothing to do with reality. Luckily for us, on PostgreSQL 9.6+ there is a `pg_stat_wal_receiver` view, which contains current values of `primary_conninfo` and `primary_slot_name`. The password field is masked through, but this is fine, because authentication happens only during opening the connection. All other parameters we compare as usual.

Another advantage of `pg_stat_wal_recevier` - it contains the current timeline, therefore on 9.6+ we don't need to use the replication connection trick if walreceiver process is alive.

If there is no walreceiver process available or it is not streaming we will stick to old methods.

tests/test_postgresql.py

@@ -250,6 +250,10 @@ class TestPostgresql(BaseTestPostgresql):
             self.p.config.write_recovery_conf({'standby_mode': 'on', 'primary_conninfo': conninfo.copy()})
             self.p.config.write_postgresql_conf()
             self.assertEqual(self.p.config.check_recovery_conf(None), (False, False))
+            with patch.object(Postgresql, 'primary_conninfo', Mock(return_value='host=1')):
+                mock_get_pg_settings.return_value['primary_slot_name'] = [
+                    'primary_slot_name', '', '', 'string', 'postmaster', self.p.config._postgresql_conf]
+                self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
 
     @patch.object(Postgresql, 'major_version', PropertyMock(return_value=120000))
     @patch.object(Postgresql, 'is_running', MockPostmaster)
@@ -267,6 +271,7 @@ class TestPostgresql(BaseTestPostgresql):
             self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
 
     @patch.object(Postgresql, 'major_version', PropertyMock(return_value=100000))
+    @patch.object(Postgresql, 'primary_conninfo', Mock(return_value='host=1'))
     def test__read_recovery_params_pre_v12(self):
         self.p.config.write_recovery_conf({'standby_mode': 'on', 'primary_conninfo': {'password': 'foo'}})
         self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
@@ -695,3 +700,9 @@ class TestPostgresql(BaseTestPostgresql):
     @patch('os.path.isfile', Mock(return_value=False))
     def test_pgpass_is_dir(self):
         self.assertRaises(PatroniException, self.setUp)
+
+    @patch.object(Postgresql, '_query', Mock(side_effect=RetryFailedError('')))
+    def test_received_timeline(self):
+        self.p.set_role('standby_leader')
+        self.p.reset_cluster_info_state()
+        self.assertRaises(PostgresConnectionException, self.p.received_timeline)