Special handling of check_recovery_conf for v12+ (#2292)

When starting as a replica it may take some time before Postgres starts accepting new connections, but meanwhile, it could happen that the leader transitioned to a different member and the `primary_conninfo` must be updated.

On pre v12 Patroni regularly checks `recovery.conf` in order to check that recovery parameters match the expectation. Starting from v12 recovery parameters were converted to GUC's and Patroni gets current values from the `pg_settings` view. The last one creates a problem when it takes more than a minute for Postgres to start accepting new connections.

Since Patroni attempts to execute at least `pg_is_in_recovery()` every HA loop, and it is raising at exception, the `check_recovery_conf()` effectively wasn't reachable until recovery is finished, but it changed when #2082 was introduced.

As a result of #2082 we got the following behavior:
1. Up to v12 (not including) everything was working as expected
2. v12 and v13 - Patroni restarting Postgres after 1m of recovery
3. v14+ - the `check_recovery_conf()` is not executed because the `replay_paused()` method raising an exception.

In order to properly handle changes of recovery parameters or leader transitioned to a different node on v12+, we will rely on the cached values of recovery parameters until Postgres becomes ready to execute queries.

Close https://github.com/zalando/patroni/issues/2289

tests/test_postgresql.py

@@ -95,7 +95,7 @@ class TestPostgresql(BaseTestPostgresql):
     @patch('subprocess.call', Mock(return_value=0))
     @patch('os.rename', Mock())
     @patch('patroni.postgresql.CallbackExecutor', Mock())
-    @patch.object(Postgresql, 'get_major_version', Mock(return_value=130000))
+    @patch.object(Postgresql, 'get_major_version', Mock(return_value=140000))
     @patch.object(Postgresql, 'is_running', Mock(return_value=True))
     def setUp(self):
         super(TestPostgresql, self).setUp()
@@ -287,6 +287,8 @@ class TestPostgresql(BaseTestPostgresql):
         mock_get_pg_settings.side_effect = Exception
         with patch('patroni.postgresql.config.mtime', mock_mtime):
             self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
+        with patch.object(Postgresql, 'is_starting', Mock(return_value=True)):
+            self.assertEqual(self.p.config.check_recovery_conf(None), (False, False))
 
     @patch.object(Postgresql, 'major_version', PropertyMock(return_value=100000))
     @patch.object(Postgresql, 'primary_conninfo', Mock(return_value='host=1'))