Avoids a potential O(n^2) performance problem when dequeueing
from very popular channels.

benchmark                old ns/op     new ns/op     delta
BenchmarkChanPopular     2563782       627201        -75.54%

Change-Id: I231aaeafea0ecd93d27b268a0b2128530df3ddd6
Reviewed-on: https://go-review.googlesource.com/1200Reviewed-by: Russ Cox <rsc@golang.org>