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

app/observatory/command/command.go

@@ -38,7 +38,7 @@ func init() {
 		sv := &service{v: s}
 		err := s.RequireFeatures(func(Observatory extension.Observatory) {
 			sv.observatory = Observatory
-		})
+		}, false)
 		if err != nil {
 			return nil, err
 		}