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>