benchmark                           old ns/op    new ns/op    delta
BenchmarkClientServerParallel          155909       154454   -0.93%
BenchmarkClientServerParallel-2         86012        82986   -3.52%
BenchmarkClientServerParallel-4         70211        55168  -21.43%
BenchmarkClientServerParallel-8         80755        47862  -40.73%
BenchmarkClientServerParallel-12        77753        51478  -33.79%
BenchmarkClientServerParallel-16        77920        50278  -35.47%
The benchmark is https://golang.org/cl/6441134
The machine is 2 x 4 HT cores (16 HW threads total).
Fixes #3946.
Now contention moves to net.pollServer.AddFD().

R=bradfitz
CC=bradfitz, dave, dsymonds, gobot, golang-dev, remyoudompheng
https://golang.org/cl/6454142