Adds ReverseProxy.BufferPool for users with sensitive allocation
requirements. Permits avoiding 32 KB of io.Copy garbage per request.

Fixes #10277

Change-Id: I5dfd58fa70a363ead4be56405e507df90d871719
Reviewed-on: https://go-review.googlesource.com/9399Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>