Avoid potential race conditions by clarifying to implemntors of the
ReverseProxy interface, the lifetime of provided http.Request structs.

Fixes #18456
Change-Id: I46aa60322226ecc3a0d30fa1ef108e504171957a
Reviewed-on: https://go-review.googlesource.com/34720Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>