Lazy initialise the worker pool (#4110)

* Lazy initialise the worker pool

* Minimize pool initialization in core tests

* Fix tests on Python 3.4

* Remove passing in thread count to mock HASS

* Tests: Allow pool by default for threaded, disable for async

* Remove JobPriority for thread pool

* Fix wrong block_till_done

* EmulatedHue: Remove unused test code

* Zigbee: do not touch hass.pool

* Init loop in add_job

* Fix core test

* Fix random sensor test

tests/components/notify/test_demo.py

@@ -111,7 +111,7 @@ class TestNotifyDemo(unittest.TestCase):
         }
 
         script.call_from_config(self.hass, conf)
-        self.hass.pool.block_till_done()
+        self.hass.block_till_done()
         self.assertTrue(len(self.events) == 1)
         assert {
             'message': 'Test 123 4',