Avoid spurious warnings at shutdown

Avoid spurious warnings at shutdown

When there are still active coroutines at interpreter shutdown, you can
see sometimes warnings such as:

Exception ignored in: <bound method Future.__del__ of <tornado.concurrent.Future object at 0x7f56a8b142e8>>
Traceback (most recent call last):
  File "/home/antoine/tornado/tornado/concurrent.py", line 338, in __del__
TypeError: 'NoneType' object is not callable

These are distracting and don't bear any useful information, so silence them.