Make GlobalConfig really global (#2935)

1. extract `GlobalConfig` class to its own module
2. make the module instantiate the `GlobalConfig` object on load and replace sys.modules with the this instance
3. don't pass `GlobalConfig` object around, but use `patroni.global_config` module everywhere.
4. move `ignore_slots_matchers`, `max_timelines_history`,  and `permanent_slots` from `ClusterConfig` to `GlobalConfig`.
5. add `use_slots` property to global_config and remove duplicated code from `Cluster` and `Postgresql.ConfigHandler`.

Besides that improve readability of couple of checks in ha.py and formatting of `/config` key when saved from patronictl.

tests/test_slots.py

@@ -6,8 +6,7 @@ import unittest
 from mock import Mock, PropertyMock, patch
 from threading import Thread
 
-from patroni import psycopg
-from patroni.config import GlobalConfig
+from patroni import global_config, psycopg
 from patroni.dcs import Cluster, ClusterConfig, Member, Status, SyncState
 from patroni.postgresql import Postgresql
 from patroni.postgresql.misc import fsync_dir
@@ -29,12 +28,12 @@ class TestSlotsHandler(BaseTestPostgresql):
     @patch.object(Postgresql, 'is_running', Mock(return_value=True))
     def setUp(self):
         super(TestSlotsHandler, self).setUp()
-        self.p._global_config = GlobalConfig({})
         self.s = self.p.slots_handler
         self.p.start()
         config = ClusterConfig(1, {'slots': {'ls': {'database': 'a', 'plugin': 'b'}, 'ls2': None}}, 1)
         self.cluster = Cluster(True, config, self.leader, Status(0, {'ls': 12345, 'ls2': 12345}),
                                [self.me, self.other, self.leadermem], None, SyncState.empty(), None, None)
+        global_config.update(self.cluster)
 
     def test_sync_replication_slots(self):
         config = ClusterConfig(1, {'slots': {'test_3': {'database': 'a', 'plugin': 'b'},
@@ -42,11 +41,12 @@ class TestSlotsHandler(BaseTestPostgresql):
                                    'ignore_slots': [{'name': 'blabla'}]}, 1)
         cluster = Cluster(True, config, self.leader, Status(0, {'test_3': 10}),
                           [self.me, self.other, self.leadermem], None, SyncState.empty(), None, None)
+        global_config.update(cluster)
         with mock.patch('patroni.postgresql.Postgresql._query', Mock(side_effect=psycopg.OperationalError)):
             self.s.sync_replication_slots(cluster, False)
         self.p.set_role('standby_leader')
         with patch.object(SlotsHandler, 'drop_replication_slot', Mock(return_value=(True, False))), \
-                patch.object(GlobalConfig, 'is_standby_cluster', PropertyMock(return_value=True)), \
+                patch.object(global_config.__class__, 'is_standby_cluster', PropertyMock(return_value=True)), \
                 patch('patroni.postgresql.slots.logger.debug') as mock_debug:
             self.s.sync_replication_slots(cluster, False)
             mock_debug.assert_called_once()
@@ -94,6 +94,7 @@ class TestSlotsHandler(BaseTestPostgresql):
                                    'ignore_slots': [{'name': 'blabla'}]}, 1)
         cluster = Cluster(True, config, self.leader, Status.empty(), [self.me, self.other, self.leadermem],
                           None, SyncState.empty(), None, None)
+        global_config.update(cluster)
 
         self.s.sync_replication_slots(cluster, False)
         with patch.object(Postgresql, '_query') as mock_query:
@@ -189,6 +190,7 @@ class TestSlotsHandler(BaseTestPostgresql):
         config = ClusterConfig(1, {'slots': {'blabla': {'type': 'physical'}, 'leader': None}}, 1)
         cluster = Cluster(True, config, self.leader, Status(0, {'blabla': 12346}),
                           [self.me, self.other, self.leadermem], None, SyncState.empty(), None, None)
+        global_config.update(cluster)
         self.s.sync_replication_slots(cluster, False)
         with patch.object(SlotsHandler, '_query', Mock(side_effect=[[('blabla', 'physical', 12345, None, None, None,
                                                                       None, None)], Exception])) as mock_query, \