The addition of Server.ConnState provides all the necessary
hooks to stop a Server gracefully, but StateNew previously
could fire concurrently with Serve exiting (as it does when
its net.Listener is closed). This previously meant one
couldn't use a WaitGroup incremented in the StateNew hook
along with calling Wait after Serve. Now you can.

Update #4674

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/70410044