Log the creation of a worker task thread (#1676)

Right now the completion of a worker task thread is logged, but the
commencement of it (with any useful contextual information) is not,
which makes it difficult to follow the journey of particular failing
record imports.

docker/worker/worker.py

@@ -617,6 +617,7 @@ def process_task(ack_id, message):
           target=self._do_process_task,
           args=(subscriber, subscription, ack_id, message, done_event),
           daemon=True)
+      logging.info('Creating task thread for %s', message)
       thread.start()
 
       done = done_event.wait(timeout=MAX_LEASE_DURATION)