net/http: provide access to the listener address an HTTP request arrived on

net/http: provide access to the listener address an HTTP request arrived on

This adds a context key named LocalAddrContextKey (for now, see #15229) to
let users access the net.Addr of the net.Listener that accepted the connection
that sent an HTTP request. This is similar to ServerContextKey which provides
access to the *Server. (A Server may have multiple Listeners)

Fixes #6732

Change-Id: I74296307b68aaaab8df7ad4a143e11b5227b5e62
Reviewed-on: https://go-review.googlesource.com/22672
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>