For both the server and the transport, return connection-level flow control in
two cases: 1) when a stream is closed with buffered data not read by the user,
or 2) when a DATA frame arrives but there the stream has since been closed.

Fixes golang/go#16481

Change-Id: Ic7404180ed04a2903e8fd6e9599a907f88b4f72e
Reviewed-on: https://go-review.googlesource.com/25231Reviewed-by: Andrew Gerrand <adg@golang.org>