net: coalesce duplicate in-flight DNS lookups

In Issue 5625, Russ says: "We should at least have a cache of
inflight lookups, so that 100 simultaneous dials of one host
name don't do the work 100x. That's easy and (assume we forget
the answer once they all get it) doesn't pose any consistency
problems. It just merges simultaneous work."

This brings in singleflight (unexported) from Google /
Camlistore, but without its tests. Maybe we should put it
somewhere in the standard library. But not now.

Update issue 5625

R=golang-dev, iant, cespare, rsc, dave, rogpeppe, remyoudompheng
CC=golang-dev
https://codereview.appspot.com/10079043

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