Runner: remove logging from signal handler.

It turns out that logging.Logger SHOULD NOT be used in signal handlers.
We are running into this elsewhere in the system so that is absolutely
no reason to risk it here.

ref: https://docs.python.org/3/library/logging.html#thread-safety

docs/history.rst

@@ -3,6 +3,14 @@
 Release History
 ===============
 
+`Next Release`_
+---------------
+- Remove logging from the signal handler.  Logger's cannot safely be used
+  from within signal handlers.  See `Thread Safety`_ in the logging module
+  documentation for details.
+
+.. _Thread Safety: https://docs.python.org/3/library/logging.html#thread-safety
+
 `1.5.0`_ (29 Jan 2018)
 ----------------------
 - Enable port reuse for Tornado versions newer than 4.3.

sprockets/http/runner.py

@@ -139,7 +139,6 @@ class Runner(object):
         iol.start()
 
     def _on_signal(self, signo, frame):
-        self.logger.info('signal %s received, stopping', signo)
         ioloop.IOLoop.instance().add_callback_from_signal(self._shutdown)
 
     def _shutdown(self):