In certain shutdown cases (from the client and/or server), the http2
Server can Push stream-specific frames on closed streams. This caused
memory leaks in the random write scheduler.

As a conservative fix for backporting, just clear the map element
whenever its queue value is empty. The map entry is re-created as
needed anyway. This isn't perfectly ideal (it adds a map+delete and
free queue put+get) in the case where a stream is open & actively
writing, but it's an easy fix for now. A future CL can optimize all
this code. It looks like there are some other good optimization
opportunities in related code anyway. But I'd rather that happen on
master and not be done in a backported change.

Updates golang/go#34636 (needs bundle to std before fixed)
Updates golang/go#33812

Change-Id: I21508ba2ebc361e8b8532d0d1cebf882e82c473c
Reviewed-on: https://go-review.googlesource.com/c/net/+/198462
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit d98b1b4)
Reviewed-on: https://go-review.googlesource.com/c/net/+/198617
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>

…k fix

This re-runs go generate with x/net checked out at CL 198617 on the
release-branch.go1.13 branch for:

   [release-branch.go1.13] http2: fix memory leak in random write scheduler

Fixes #34636
Updates #33812

Change-Id: Ibce630c6c7ffe43ff760d2ad40b44731c07ba870
Reviewed-on: https://go-review.googlesource.com/c/go/+/198897
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>