The original signature accept a boolean, and it feel like a little
verbose, since when people pass in this option, he/she always want to
pass a `true`.
Now if input `wg` is nil, it has same effect as passing `true` in
original code. Furthermore, if user want's finer grained control during
shutdown, one can pass in a predefined `wg`, so that server will wait
against it during shutdown.
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