http2: write Transport bodies

Some polish and testing remains, but this should be the bulk of the
Transport work that was remaining.

There's one notable (but easy) TODO about sending WINDOW_UPDATE frames
every few hundred MB or so, but this is a checkpoint.

Updates golang/go#6891

Change-Id: Iced9850804bf2c069c75118895ee7c3750ba31b5
Reviewed-on: https://go-review.googlesource.com/16310
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
3 files changed