Improve leader watch functionality (#356)

Previously replicas were always watching for leader key (even if the
postgres was not in the running there). It was not a big issue, but it
was not possible to interrupt such watch in cases if the postgres
started up or stopped successfully. Also it was delaying update_member
call and we had kind of stale information in DCS up to `loop_wait`
seconds. This commit changes such behavior. If the async_executor is
busy by starting/stopping or restarting postgres we will not watch for
leader key but waiting for event from async_executor up to `loop_wait`
seconds. Async executor will fire such event only in case if the
function it was calling returned something what could be evaluated to
boolean True.

Such functionality is really needed to change the way how we are making
decision about necessity of pg_rewind. It will require to have a local
postgres running and for us it is really important to get such
notification as soon as possible.

tests/test_async_executor.py

@@ -8,7 +8,7 @@ from threading import Thread
 class TestAsyncExecutor(unittest.TestCase):
 
     def setUp(self):
-        self.a = AsyncExecutor()
+        self.a = AsyncExecutor(Mock())
 
     @patch.object(Thread, 'start', Mock())
     def test_run_async(self):