Log: Add microseconds for golang's standard logger

Completes https://github.com/XTLS/Xray-core/commit/5679d717eef23f48d4f60a0a34684f46c478e053

main/run.go

@@ -45,6 +45,7 @@ The -dump flag tells Xray to print the merged config.
 
 func init() {
 	cmdRun.Run = executeRun // break init loop
+	log.SetFlags(log.Ldate | log.Ltime | log.Lmicroseconds)
 }
 
 var (