Fixes s.concurrency

This version fixes server concurrency counter for cases, when you use ListenAndServe, and not ServeConn manually.

For example, in previous versions if your server is started by ListenAndServe, you will have GetCurrentConcurrency() returning constant zero. This version is fixing this. You can find corresponding PR here.

This is considered a minor update, to allow users some time to update, if they relied on previous behavior.

Feel free to report any bugs here. Thank you.