Closes rust-lang#6947

This changes the lint to allow futures which are not `Send` as a result
of a generic type parameter not having a `Send` bound and only lint
futures that are always `!Send` for any type, which I believe is the
more useful behavior (like the comments in the linked issue explain).

This is still only a heuristic (I'm not sure if there's a more general
way to do this), but it should cover the common cases I could think of
(including the code examples in the linked issue)

changelog: [`future_not_send`]: allow conditional `Send` futures