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_config.py

@@ -5,7 +5,11 @@ import io
 
 from copy import deepcopy
 from mock import MagicMock, Mock, patch
-from patroni.config import Config, ConfigParseError, GlobalConfig
+
+from patroni import global_config
+from patroni.config import ClusterConfig, Config, ConfigParseError
+
+from .test_ha import get_cluster_initialized_with_only_leader
 
 
 class TestConfig(unittest.TestCase):
@@ -248,4 +252,6 @@ class TestConfig(unittest.TestCase):
     def test_global_config_is_synchronous_mode(self):
         # we should ignore synchronous_mode setting in a standby cluster
         config = {'standby_cluster': {'host': 'some_host'}, 'synchronous_mode': True}
-        self.assertFalse(GlobalConfig(config).is_synchronous_mode)
+        cluster = get_cluster_initialized_with_only_leader(cluster_config=ClusterConfig(1, config, 1))
+        test_config = global_config.from_cluster(cluster)
+        self.assertFalse(test_config.is_synchronous_mode)