Profiling note

1. A little bit off topic to profiling. I learnt that putting os.Signal channel means that when it receive stop signal (ctrl + c), it will fill the done channel and then it will shutdown the server
2. For CPU profiling, we can create CPU profiling file, then we do pprof.StartCPUProfile(file). After we finish running our web server, we can run go tool pprof <cpu profile file path>, then we can enter top 10 to get top 10 longest bottleneck