http2: add support for Transport reading trailers from the server
http2: add support for Transport reading trailers from the server

Tests in the main go repo.

Updates golang/go#13557

Change-Id: Ide7be9e24b603008faf9f3ea55e936b86ea1c42b
Reviewed-on: https://go-review.googlesource.com/17930
Reviewed-by: Blake Mizerany <9064db5bb6ad06c7c3afee8dc1334281c314ed63@gmail.com>

cesanta/goxnet,golang/net,DramaFever/golang.org-x-net,cmars/tools,cesanta/goxnet,albertjin/golang-net,albertjin/golang-net,cmars/tools,DramaFever/golang.org-x-net,cmars/tools,cmars/tools