http2: support SetReadDeadline, SetWriteDeadline, FlushError
http2: support SetReadDeadline, SetWriteDeadline, FlushError

Add support for ResponseController methods to set read/write deadlines
and flush with an error return.

Change-Id: I63059d13bef28dc32a4b8b75901eb1dd56176f32
Reviewed-on: https://go-review.googlesource.com/c/net/+/446335
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

golang/net