Refactor write_sync_state() (#2669)

Make it return the new `SyncState` object in order to avoid reading the new cluster state in the Ha.process_sync_replication().

Now it is a small optimization, but it will become very handy in the quorum commit feature.

tests/test_etcd.py

@@ -338,7 +338,7 @@ class TestEtcd(unittest.TestCase):
         self.assertTrue(self.etcd.watch(None, 1))
 
     def test_sync_state(self):
-        self.assertFalse(self.etcd.write_sync_state('leader', None))
+        self.assertIsNone(self.etcd.write_sync_state('leader', None))
         self.assertFalse(self.etcd.delete_sync_state())
 
     def test_set_history_value(self):