Problem: Random failure for event handler test (#2423)

Solution: Random failure of test
`tests/test_events.py::test_event_handler_raises_when_called_after_start` seem
to be because of `get_nowait` which returns way to quickly even when the queue
has data. Increasing timeout seems to fix the issue.

bigchaindb/events.py

@@ -67,7 +67,7 @@ def get_subscriber_queue(self, event_types=None):
         """
 
         try:
-            self.started_queue.get_nowait()
+            self.started_queue.get(timeout=1)
             raise RuntimeError('Cannot create a new subscriber queue while Exchange is running.')
         except Empty:
             pass