Update #4674

This allows for all sorts of graceful shutdown policies,
without picking a policy (e.g. lameduck period) and without
adding lots of locking to the server core. That policy and
locking can be implemented outside of net/http now.

LGTM=adg
R=golang-codereviews, josharian, r, adg, dvyukov
CC=golang-codereviews
https://golang.org/cl/69260044