Replace concurrent.futures.Future with tornado.concurrent.Future

[yurishkuro]

Signed-off-by: Yuri Shkuro ys@uber.com

[coveralls]

Coverage decreased (-0.05%) to 95.322% when pulling 6f741a6 on remove-futures-dependendcy into 4200bdb on master.

[black-adder]

sooo how does this work? 'futures;python_version<"3"' requires a higher version of setuptools but somehow tornado.concurrent.Future doesn't? Are we adding a indirect dependence?

[yurishkuro]

@black-adder concurrent.futures is standard in Py3 and futures is a backport of that into Py2 (that's why we can't depend on it in Py3, that module doesn't even compile w/ Py3 - although it could've)

On the other hand, tornado.concurrent.Future is a separate implementation that exists both in py2 and py3 (it might be just an alias to the official Future in py3).