runtime: convert timers to Go

LGTM=rsc
R=golang-codereviews, ruiu, rsc, daniel.morsing
CC=golang-codereviews, khr
https://codereview.appspot.com/123700044
runtime: add comment
Explain why it's safe to allocate chans with flagNoScan.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/125510045