Improve patronictl reinit (#576)

Make it possible to cancel a running task if you want to reinitialize replica.
There are two possible ways to trigger it:
1. patronictl will ask whether you want to cancel already running task if an attempt to trigger reinitialize has failed
2. if you are using `--force` argument with `patronictl reinit`

tests/test_async_executor.py

@@ -8,7 +8,7 @@ from threading import Thread
 class TestAsyncExecutor(unittest.TestCase):
 
     def setUp(self):
-        self.a = AsyncExecutor(Mock())
+        self.a = AsyncExecutor(Mock(), Mock())
 
     @patch.object(Thread, 'start', Mock())
     def test_run_async(self):
@@ -17,6 +17,12 @@ class TestAsyncExecutor(unittest.TestCase):
     def test_run(self):
         self.a.run(Mock(side_effect=Exception()))
 
+    def test_cancel(self):
+        self.a.cancel()
+        self.a.schedule('foo')
+        self.a.cancel()
+        self.a.run(Mock())
+
 
 class TestCriticalTask(unittest.TestCase):