engine: graph: Use an atomic bool instead of a mutex

The isStateOK variable can be accessed concurrently as these are
supposed to be "benign" races. As such, they need to be labelled as such
so that we don't hit some undefined compiler behaviour.

Here are five good references relating to "benign" data races in golang.

1) https://web.archive.org/web/20181022150257/https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong

2) https://go.dev/ref/mem - "Informal Overview" section.

3) https://docs.oracle.com/cd/E19205-01/820-0619/gecqt/index.html

4) https://www.usenix.org/legacy/event/hotpar11/tech/final_files/Boehm.pdf

5) https://go.dev/doc/articles/race_detector

TL;DR: wrap your benign races with sync/atomic or eliminate them.

engine/graph/actions.go

@@ -165,7 +165,7 @@ func (obj *Engine) Process(ctx context.Context, vertex pgraph.Vertex) error {
 	// Check cached state, to skip CheckApply, but can't skip if refreshing!
 	// If the resource doesn't implement refresh, skip the refresh test.
 	// FIXME: if desired, check that we pass through refresh notifications!
-	if (!refresh || !isRefreshableRes) && obj.state[vertex].isStateOK {
+	if (!refresh || !isRefreshableRes) && obj.state[vertex].isStateOK.Load() { // mutex RLock/RUnlock
 		checkOK, err = true, nil
 
 	} else if noop && (refresh && isRefreshableRes) { // had a refresh to do w/ noop!
@@ -193,7 +193,9 @@ func (obj *Engine) Process(ctx context.Context, vertex pgraph.Vertex) error {
 	// if CheckApply ran without noop and without error, state should be good
 	if !noop && err == nil { // aka !noop || checkOK
 		obj.state[vertex].tuid.StartTimer()
-		obj.state[vertex].isStateOK = true // reset
+		//obj.state[vertex].mutex.Lock()
+		obj.state[vertex].isStateOK.Store(true) // reset
+		//obj.state[vertex].mutex.Unlock()
 		if refresh {
 			obj.SetUpstreamRefresh(vertex, false) // refresh happened, clear the request
 			if isRefreshableRes {