Fix access to map from multiple goroutines

[Pliner]

https://golang.org/ref/mem#tmp_10

Another incorrect idiom is busy waiting for a value, as in:

var a string
var done bool

func setup() {
	a = "hello, world"
	done = true
}

func main() {
	go setup()
	for !done {
	}
	print(a)
}

As before, there is no guarantee that, in main, observing the write to done implies observing the write to a, so this program could print an empty string too. Worse, there is no guarantee that the write to done will ever be observed by main, since there are no synchronization events between the two threads. The loop in main is not guaranteed to finish.

[coveralls]

Coverage remained the same at 83.12% when pulling 1b618d9 on fix-access-to-map-from-multiple-goroutines into 80d479a on develop.