tornado: Use spiffy new call_later rather than add_timeout.

This method was new in Tornado 4.0.  It saves us from having to get
the time ourselves and do the arithmetic -- which not only makes the
code a bit shorter, but also easier to get right.  Tornado docs (see
http://www.tornadoweb.org/en/stable/ioloop.html) say we should have
been getting the time from `ioloop.time()` rather than hardcoding
`time.time()`, because the loop could e.g. be running on the
`time.monotonic()` clock.

tools/run-dev.py

@@ -380,7 +380,7 @@ def shutdown_handler(*args, **kwargs):
     # type: (*Any, **Any) -> None
     io_loop = IOLoop.instance()
     if io_loop._callbacks:
-        io_loop.add_timeout(time.time() + 1, shutdown_handler)
+        io_loop.call_later(1, shutdown_handler)
     else:
         io_loop.stop()