Routines and logic to preform a concurrent stack scan of go-routines.
This CL excersizes most of the functionality needed. The
major exception being that it does not scan running goroutines.
After doing the scans it relies on a STW to finish the GC, including
rescanning the stacks. It is intended to achieve correctness,
performance will follow.

LGTM=rsc
R=golang-codereviews, rsc
CC=dvyukov, golang-codereviews
https://golang.org/cl/156580043