The OCSP response is currently only exposed via a method on Conn,
which makes it inaccessible when using wrappers like net/http. The
ConnectionState structure is typically available even when using
wrappers and contains many of the other handshake details, so this
change exposes the stapled OCSP response in that structure.

Change-Id: If8dab49292566912c615d816321b4353e711f71f
Reviewed-on: https://go-review.googlesource.com/9361Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>