In general, clean up and simplify the handling of frame writing from
handler goroutines.  Always select on streams closing, and don't try
to pass around and re-use channels. It was too confusing. Instead,
reuse channels in a very local manner that's easy to reason about.

Thanks to Github user @pabbott0 (who has signed the Google CLA) for
the initial bug report and test cases.

Fixes bradfitz/http2#45

Change-Id: Ib72a87cb6e33a4bb118ae23d765ba594e9182ade
Reviewed-on: https://go-review.googlesource.com/15820Reviewed-by: Andrew Gerrand <adg@golang.org>