Currently, sysmon waits 60 ms during idle before relaxing. This is
primarily to avoid reducing the precision of short-duration timers. Of
course, if there are no short-duration timers, this wastes 60 ms
running the timer at high resolution.

Improve this by instead inspecting the time until the next timer fires
and relaxing the timer resolution immediately if the next timer won't
fire for a while.

Updates #20937.

Change-Id: If4ad0a565b65a9b3e8c4cdc2eff1486968c79f24
Reviewed-on: https://go-review.googlesource.com/47833
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>