Events bok-choy multiprocessing support.

Stop clearing the entire mongo collection with every mixin instantiation.
Instead, reset the clock for the particular test.

With this approach, filters must be stricter, for example, according
to username or user id.

common/test/acceptance/tests/helpers.py

@@ -351,7 +351,7 @@ class EventsTestMixin(TestCase):
     def setUp(self):
         super(EventsTestMixin, self).setUp()
         self.event_collection = MongoClient()["test"]["events"]
-        self.reset_event_tracking()
+        self.start_time = datetime.now()
 
     def reset_event_tracking(self):
         """Drop any events that have been collected thus far and start collecting again from scratch."""