Unify global recent / global clock usage.

[tobz]

During the work to expose more free functions on Instant and add mock ability, we created a small rift between Clock and Instant.

Primarily, we have three issues:

• Instant::recent will fallback to the global clock, while Clock::recent won't
• Instant::recent is an acquire load, while Clock::recent is a relaxed load
• quanta::set_recent and Clock::upkeep aren't DRY (small problem, but still)

We should unify these codepaths so that we're limiting the chance for differences in behavior, as well as switching entirely to relaxed loads for "recent" time to ensure maximum performance.