Don't cache anything in Zookeeper implementation (#2909)

Cache creates a lot of problems and prevents implementing a feature of automatic retention of physical replication slots for members with configurable retention policy.

Just read the entire cluster from Zookeeper instead and use watchers only for the `/leader` and `/config` keys.

tests/test_zookeeper.py

@@ -1,13 +1,13 @@
 import select
 import unittest
 
-from kazoo.client import KazooClient, KazooState
+from kazoo.client import KazooClient
 from kazoo.exceptions import NoNodeError, NodeExistsError
 from kazoo.handlers.threading import SequentialThreadingHandler
-from kazoo.protocol.states import KeeperState, ZnodeStat
+from kazoo.protocol.states import KeeperState, WatchedEvent, ZnodeStat
 from kazoo.retry import RetryFailedError
 from mock import Mock, PropertyMock, patch
-from patroni.dcs.zookeeper import Cluster, Leader, PatroniKazooClient, \
+from patroni.dcs.zookeeper import Cluster, PatroniKazooClient, \
     PatroniSequentialThreadingHandler, ZooKeeper, ZooKeeperError
 
 
@@ -152,9 +152,6 @@ class TestZooKeeper(unittest.TestCase):
                              'name': 'foo', 'ttl': 30, 'retry_timeout': 10, 'loop_wait': 10,
                              'set_acls': {'CN=principal2': ['ALL']}})
 
-    def test_session_listener(self):
-        self.zk.session_listener(KazooState.SUSPENDED)
-
     def test_reload_config(self):
         self.zk.reload_config({'ttl': 20, 'retry_timeout': 10, 'loop_wait': 10})
         self.zk.reload_config({'ttl': 20, 'retry_timeout': 10, 'loop_wait': 5})
@@ -176,15 +173,6 @@ class TestZooKeeper(unittest.TestCase):
         self.zk._cluster_loader(self.zk.client_path(''))
 
     def test_get_cluster(self):
-        cluster = self.zk.get_cluster(True)
-        self.assertIsInstance(cluster.leader, Leader)
-        self.zk.status_watcher(None)
-        self.zk.get_cluster()
-        self.zk.touch_member({'foo': 'foo'})
-        self.zk._name = 'bar'
-        self.zk.status_watcher(None)
-        with patch.object(ZooKeeper, 'get_node', Mock(side_effect=Exception)):
-            self.zk.get_cluster()
         cluster = self.zk.get_cluster()
         self.assertEqual(cluster.last_lsn, 500)
 
@@ -295,3 +283,7 @@ class TestZooKeeper(unittest.TestCase):
 
     def test_set_history_value(self):
         self.zk.set_history_value('{}')
+
+    def test_watcher(self):
+        self.zk._watcher(WatchedEvent('', '', ''))
+        self.assertTrue(self.zk.watch(1, 1))