asyncio: sync with Tulip

* Tulip issue #182: Improve logs of BaseEventLoop._run_once()

  - Don't log non-blocking poll
  - Only log polling with a timeout if it gets events or if it timed out after
    more than 1 second.

* Fix some pyflakes warnings: remove unused imports
(Merge 3.4) asyncio: sync with Tulip

* Tulip issue #182: Improve logs of BaseEventLoop._run_once()

  - Don't log non-blocking poll
  - Only log polling with a timeout if it gets events or if it timed out after
    more than 1 second.

* Fix some pyflakes warnings: remove unused imports
asyncio: enhance documentation of platform support
(Merge 3.4) asyncio: enhance documentation of platform support