http2/h2demo: set externalTrafficPolicy to preserve client IPs
http2/h2demo: set externalTrafficPolicy to preserve client IPs

So https://http2.golang.org/reqinfo has a RemoteAddr of the client's
actual IP address.

Change-Id: I9db52b2f451ad937d013dd78078e4228e732ffe0
Reviewed-on: https://go-review.googlesource.com/99235
Reviewed-by: David Anderson <93b19e97294520db5d300a69ac27bdbeaec3051d@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net