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/router/command/command.go

@@ -135,7 +135,7 @@ func (s *service) Register(server *grpc.Server) {
 		vCoreDesc := RoutingService_ServiceDesc
 		vCoreDesc.ServiceName = "v2ray.core.app.router.command.RoutingService"
 		server.RegisterService(&vCoreDesc, rs)
-	}))
+	}, false))
 }
 
 func init() {