Also remove some stale TODOs and finish one of the TODOs, ignoring
more incoming frames when the connection is in GOAWAY mode.

Also, fix independently-broken transport test from a change in std:
https://golang.org/cl/31595

Updates golang/go#14204

Change-Id: Iae8b02248464d613979c30d8f86eacb329cd262f
Reviewed-on: https://go-review.googlesource.com/31727Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>