allp < timers has not been necessary since CL 258303.
sched < timers was implied by allp < timers, and that
was still necessary, but only when the world is stopped.
Rewrite the code to avoid that lock since the world is stopped.
Now timers and timer are independent of the scheduler,
so they could call into the scheduler (for example to ready
a goroutine) if we wanted them to.
Change-Id: I12a93013c98e51c9e2f2148175b02afce8384a59
Reviewed-on: https://go-review.googlesource.com/c/go/+/568337
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>