Watchdog unit-tests, bugfixes and questions (#449)

Implement missing unit-tests for and drop unused code

tests/test_async_executor.py

@@ -1,7 +1,7 @@
 import unittest
 
 from mock import Mock, patch
-from patroni.async_executor import AsyncExecutor
+from patroni.async_executor import AsyncExecutor, CriticalTask
 from threading import Thread
 
 
@@ -16,3 +16,11 @@ class TestAsyncExecutor(unittest.TestCase):
 
     def test_run(self):
         self.a.run(Mock(side_effect=Exception()))
+
+
+class TestCriticalTask(unittest.TestCase):
+
+    def test_completed_task(self):
+        ct = CriticalTask()
+        ct.complete(1)
+        self.assertFalse(ct.cancel())