return_future and friends now pass the result, not the Future, to the callback.

Functions and decorators that take an optional callback and return
a future (return_future, gen.concurrent, and run_executor) no longer pass
the Future object to the callback.  This results in somewhat less flexible
error handling, but is more consistent with prevailing practice without
Futures.
Bump version to 2.4.post4
Allow the callback in @return_future to be run with no arguments.