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/proxyman/command/command.go

@@ -177,7 +177,7 @@ func (s *service) Register(server *grpc.Server) {
 	common.Must(s.v.RequireFeatures(func(im inbound.Manager, om outbound.Manager) {
 		hs.ihm = im
 		hs.ohm = om
-	}))
+	}, false))
 	RegisterHandlerServiceServer(server, hs)
 
 	// For compatibility purposes