util.CallbackManager: use sets instead of lists

- to gracefully take duplicate calls of register_callbacks(): should be idempotent now
- as a side-effect, the order of the callbacks is changed and not guaranteed
  - not like anyone should have been relying on it before though

tests/test_callbackmgr.py

@@ -49,13 +49,13 @@ class TestCallbackMgr(ElectrumTestCase):
         el2.start()
         self.assertEqual(4, _count_all_callbacks())
         el1.start()
-        self.assertEqual(6, _count_all_callbacks())
-        el1.stop()
         self.assertEqual(4, _count_all_callbacks())
         el1.stop()
         self.assertEqual(2, _count_all_callbacks())
         el1.stop()
         self.assertEqual(2, _count_all_callbacks())
+        el1.stop()
+        self.assertEqual(2, _count_all_callbacks())
         el2.stop()
         self.assertEqual(0, _count_all_callbacks())