http2, http2/hpack: add limit on sum of header block fragments

Fixes golang/go#12916

Change-Id: I085ebc1d3f851d7c1b689a715c4f2fe85be4608a
Reviewed-on: https://go-review.googlesource.com/15821
Reviewed-by: Andrew Gerrand <adg@golang.org>
4 files changed