Add a benchmark for coroutine processing.
tornado.concurrent.Future is now always thread-unsafe.

This improves performance and provides more consistent semantics
(independent of the presence or absence of the concurrent.futures
package).

The parts of Tornado that accept Futures will accept both Tornado's
thread-unsafe Futures and the thread-safe concurrent.futures.Future.
Pivot tornado.gen to use Futures natively.

This improves performance of applications using @coroutine and Futures,
at the expense of some performance degradation for the older YieldPoint
interfaces.