Lighten contention without preventing further improvements on pollservers.
Connections are spread over Min(GOMAXPROCS, NumCPU, 8) pollserver instances.

Median of 10 runs, 4 cores @ 3.4GHz, amd/linux-3.2:

BenchmarkTCPOneShot                171917 ns/op   175194 ns/op      1.91%
BenchmarkTCPOneShot-2              101413 ns/op   109462 ns/op      7.94%
BenchmarkTCPOneShot-4               91796 ns/op    35712 ns/op    -61.10%
BenchmarkTCPOneShot-6               90938 ns/op    30607 ns/op    -66.34%
BenchmarkTCPOneShot-8               90374 ns/op    29150 ns/op    -67.75%
BenchmarkTCPOneShot-16             101089 ns/op   111526 ns/op     10.32%

BenchmarkTCPOneShotTimeout         174986 ns/op   178606 ns/op      2.07%
BenchmarkTCPOneShotTimeout-2       101585 ns/op   110678 ns/op      8.95%
BenchmarkTCPOneShotTimeout-4        91547 ns/op    35931 ns/op    -60.75%
BenchmarkTCPOneShotTimeout-6        91496 ns/op    31019 ns/op    -66.10%
BenchmarkTCPOneShotTimeout-8        90670 ns/op    29531 ns/op    -67.43%
BenchmarkTCPOneShotTimeout-16      101013 ns/op   106026 ns/op      4.96%

BenchmarkTCPPersistent              51731 ns/op    53324 ns/op      3.08%
BenchmarkTCPPersistent-2            32888 ns/op    30678 ns/op     -6.72%
BenchmarkTCPPersistent-4            25751 ns/op    15595 ns/op    -39.44%
BenchmarkTCPPersistent-6            26737 ns/op     9805 ns/op    -63.33%
BenchmarkTCPPersistent-8            26850 ns/op     9730 ns/op    -63.76%
BenchmarkTCPPersistent-16          104449 ns/op   102838 ns/op     -1.54%

BenchmarkTCPPersistentTimeout       51806 ns/op    53281 ns/op      2.85%
BenchmarkTCPPersistentTimeout-2     32956 ns/op    30895 ns/op     -6.25%
BenchmarkTCPPersistentTimeout-4     25994 ns/op    18111 ns/op    -30.33%
BenchmarkTCPPersistentTimeout-6     26679 ns/op     9846 ns/op    -63.09%
BenchmarkTCPPersistentTimeout-8     26810 ns/op     9727 ns/op    -63.72%
BenchmarkTCPPersistentTimeout-16   101652 ns/op   104410 ns/op      2.71%

R=rsc, dvyukov, dave, mikioh.mikioh, bradfitz, remyoudompheng
CC=golang-dev
https://golang.org/cl/6496054