runtime: rename timers fields for clarity

runtime: rename timers fields for clarity

These names were copied over from the p field names,
but now that they are part of the timers type they can use
shorter names that make the relationship clearer.

timer0When -> minWhen
timerModifiedEarliest -> minNextWhen

This code change is only the renaming.

Change-Id: I1c0adc0b3a1289d35639619d5c945585b2d81a9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/564975
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>