Updates http2 to x/net/http2 git rev 906cda9 for:

http2: add configurable knobs for the server's receive window
https://golang.org/cl/37226

http2/hpack: speedup Encoder.searchTable
https://golang.org/cl/37406

http2: Add opt-in option to Framer to allow DataFrame struct reuse
https://golang.org/cl/34812

http2: replace fixedBuffer with dataBuffer
https://golang.org/cl/37400

http2/hpack: remove hpack's constant time string comparison
https://golang.org/cl/37394

Updates golang/go#16512
Updates golang/go#18404

Change-Id: I1ad7c95c404ead4ced7f85af061cf811b299a288
Reviewed-on: https://go-review.googlesource.com/37500Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>