Advanced validation of PostgreSQL parameters (#1674)

So far Patroni was performing a comparison of the old value (in the `pg_settings`) with the new value (from Patroni configuration or from DCS) in order to figure out if reload or restart is required when the parameter has been changed. If the given parameter was missing in the `pg_settings` Patroni was ignoring it and not writing into the `postgresql.conf`.

In case if Postgres is not running, no validation has been performed and parameters and values were written into the config as it is.

It is not a very common mistake, but people tend to mistype parameter names or values.
Also, it happens that some parameters are removed in specific Postgres versions and some new are added (e.g. `checkpoint_segments` replaced with `min_wal_size` and `max_wal_size` in 9.5 or` wal_keep_segments` was replaced with `wal_keep_size` in 13).

Writing nonexistent parameters or invalid values into the `postgresql.conf` makes postgres unstartable.
This change doesn't solve the issue 100%, but at least approaching this goal very close.

tests/test_postgresql.py

@@ -272,10 +272,10 @@ class TestPostgresql(BaseTestPostgresql):
     @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.p.config.write_recovery_conf({'standby_mode': 'off', 'primary_conninfo': {'password': 'foo'}})
         self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
         self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
-        self.p.config.write_recovery_conf({'standby_mode': '\n'})
+        self.p.config.write_recovery_conf({'restore_command': '\n'})
         with patch('patroni.postgresql.config.mtime', mock_mtime):
             self.assertEqual(self.p.config.check_recovery_conf(None), (True, True))
 
@@ -728,3 +728,8 @@ class TestPostgresql(BaseTestPostgresql):
         self.p.set_role('standby_leader')
         self.p.reset_cluster_info_state()
         self.assertRaises(PostgresConnectionException, self.p.received_timeline)
+
+    def test__write_recovery_params(self):
+        self.p.config._write_recovery_params(Mock(), {'pause_at_recovery_target': 'false'})
+        with patch.object(Postgresql, 'major_version', PropertyMock(return_value=90400)):
+            self.p.config._write_recovery_params(Mock(), {'recovery_target_action': 'PROMOTE'})