AFAICT, activeRes serves no real purpose. It is used in just two ways:

- To reduce the number of calls to closeIfIdle, which reduces the number
  of acquires of cc.mu when there are many concurrent streams. I dug
  through the CL history and could not find any benchmarks showing that
  this is necessary.

- To avoid redundant calls to cs.bufPipe.CloseWithError(err) when a read
  loop is shutdown. This is unnecessary, since redundant CloseWithError
  calls are ignored.

Since there isn't a good reason to have activeRes, the simplest way to
fix the leak is to remove activeRes entirely.

Updates golang/go#21543

Change-Id: I1d1d2dc6c946425a2772c8bf71436707021ac269
Reviewed-on: https://go-review.googlesource.com/80137Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>