Improve debug support for asyncio futures

Improve debug support for asyncio futures

When in debug mode, asyncio returns the instantiation place of a Future
and places it in its repr(), for example:

  <Future finished result=None created at /home/antoine/distributed/distributed/worker.py:1223>

This is useful when asyncio logs cancelled futures or futures that were not
waited upon after erroring out.

However, when using @gen.coroutine, we need to fix the recorded stack trace
otherwise the display is much less useful:

  <Future finished result=None created at /home/antoine/tornado/tornado/gen.py:295>