Audit std for places where a Send + 'static bound can be relaxed to just Send

[huonw]

Once #22319 lands, rust-lang/rfcs#458 will be implemented and so Send will no longer imply 'static. Most users of Send in the standard library were changed to Send + 'static to conservatively ensure safety, but many can likely lose the 'static bound.

In #22319 (specifically, in comments on huonw@a85afe6), @nikomatsakis pointed out these as likely candidates:

• std::sync::mpsc Remove 'static bound from sync::mpsc, Mutex and RwLock. #22574
• std::sync::future
• std::sync::mutex Remove 'static bound from sync::mpsc, Mutex and RwLock. #22574
• term

[huonw]

I think this is done.