net/http: use atomic.Value for Transport's alternate protocol map

net/http: use atomic.Value for Transport's alternate protocol map

Fix an old TODO and use atomic.Value for holding the Transport's
alternate protocol map. It is very frequently accessed and almost
never set or updated.

Change-Id: Ic5a71c504bdac76678114c6390d1fc0673e07aa9
Reviewed-on: https://go-review.googlesource.com/29967
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>