Quorum based failover (#2668)

To enable quorum commit:
```diff
$ patronictl.py edit-config
--- 
+++ 
@@ -5,3 +5,4 @@
   use_pg_rewind: true
 retry_timeout: 10
 ttl: 30
+synchronous_mode: quorum

Apply these changes? [y/N]: y
Configuration changed
```

By default Patroni will use `ANY 1(list,of,stanbys)` in `synchronous_standby_names`. That is, only one node out of listed replicas will be used for quorum.
If you want to increase the number of quorum nodes it is possible to do it with:
```diff
$ patronictl edit-config
--- 
+++ 
@@ -6,3 +6,4 @@
 retry_timeout: 10
 synchronous_mode: quorum
 ttl: 30
+synchronous_node_count: 2

Apply these changes? [y/N]: y
Configuration changed
```

Good old `synchronous_mode: on` is still supported.

Close https://github.com/patroni/patroni/issues/664
Close https://github.com/zalando/patroni/pull/672

tests/test_etcd.py

@@ -344,7 +344,7 @@ class TestEtcd(unittest.TestCase):
         self.assertTrue(self.etcd.watch(None, 1))
 
     def test_sync_state(self):
-        self.assertIsNone(self.etcd.write_sync_state('leader', None))
+        self.assertIsNone(self.etcd.write_sync_state('leader', None, 0))
         self.assertFalse(self.etcd.delete_sync_state())
 
     def test_set_history_value(self):