[WIP] Standby cluster implementation (#679)

Implementation of "standby cluster" described in #657. Standby cluster consists
of a "standby leader", that replicates from a "remote master" (which is not a
part of current patroni cluster and can be anywhere), and cascade replicas,
that replicate from the corresponding standby leader. "Standby leader" behaves
pretty much like a regular leader, which means that it holds a leader lock in
DSC, in case if disappears there will be an election of a new "standby
leader".
One can define such a cluster using the section "standby_cluster" in patroni
config file. This section provides parameters for standby cluster, that will be
applied only once during bootstrap and can be changed only through DSC.

tests/test_config.py

@@ -23,7 +23,7 @@ class TestConfig(unittest.TestCase):
     def test_set_dynamic_configuration(self):
         with patch.object(Config, '_build_effective_configuration', Mock(side_effect=Exception)):
             self.assertIsNone(self.config.set_dynamic_configuration({'foo': 'bar'}))
-        self.assertTrue(self.config.set_dynamic_configuration({'synchronous_mode': True}))
+        self.assertTrue(self.config.set_dynamic_configuration({'synchronous_mode': True, 'standby_cluster': {}}))
 
     def test_reload_local_configuration(self):
         os.environ.update({