Core: Add mutex to injection resolution (#4206)

* Revert "Add RequireFeaturesAsync() that works regardless order of app init"

* Add mutex to injection resolution

- Turns out we already support async DI resolution regardless of feature ordering
Previous code contain a race condition causing some resolution is lost
- Note that the new mutex cover s.pendingResolutions and s.features
but must not cover callbackResolution() due to deadlock
- Refactor some method names and simplify code

* Add OptionalFeatures injection

For example OptionalFeatures() is useful for fakedns module

core/xray_test.go

@@ -30,7 +30,7 @@ func TestXrayDependency(t *testing.T) {
 			t.Error("expected dns client fulfilled, but actually nil")
 		}
 		wait <- true
-	})
+	}, false)
 	instance.AddFeature(localdns.New())
 	<-wait
 }