Convert states to enums (#3293)

- Postgresql._state
- pg_isready state

tests/test_ha.py

@@ -18,6 +18,7 @@ from patroni.postgresql.bootstrap import Bootstrap
 from patroni.postgresql.callback_executor import CallbackAction
 from patroni.postgresql.cancellable import CancellableSubprocess
 from patroni.postgresql.config import ConfigHandler
+from patroni.postgresql.misc import PostgresqlState
 from patroni.postgresql.postmaster import PostmasterProcess
 from patroni.postgresql.rewind import Rewind
 from patroni.postgresql.slots import SlotsHandler
@@ -211,7 +212,7 @@ class TestHa(PostgresInit):
     @patch.object(Config, '_load_cache', Mock())
     def setUp(self):
         super(TestHa, self).setUp()
-        self.p.set_state('running')
+        self.p.set_state(PostgresqlState.RUNNING)
         self.p.set_role('replica')
         self.p.postmaster_start_time = MagicMock(return_value=str(postmaster_start_time))
         self.p.can_create_replica_without_replication_connection = MagicMock(return_value=False)
@@ -678,7 +679,7 @@ class TestHa(PostgresInit):
         self.p.restart = Mock(return_value=None)
         self.assertEqual(self.ha.restart({}), (False, 'postgres is still starting'))
         self.p.restart = false
-        self.assertEqual(self.ha.restart({}), (False, 'restart failed'))
+        self.assertEqual(self.ha.restart({}), (False, PostgresqlState.RESTART_FAILED))
         self.ha.cluster = get_cluster_initialized_with_leader()
         self.ha._async_executor.schedule('reinitialize')
         self.assertEqual(self.ha.restart({}), (False, 'reinitialize already in progress'))