Moves the network poller from net package into runtime.

benchmark                           old ns/op    new ns/op    delta
BenchmarkTCP4OneShot                   316386       287061   -9.27%
BenchmarkTCP4OneShot-2                 339822       313424   -7.77%
BenchmarkTCP4OneShot-3                 330057       306589   -7.11%
BenchmarkTCP4OneShotTimeout            341775       287061  -16.01%
BenchmarkTCP4OneShotTimeout-2          380835       295849  -22.32%
BenchmarkTCP4OneShotTimeout-3          398412       328070  -17.66%
BenchmarkTCP4Persistent                 40622        33392  -17.80%
BenchmarkTCP4Persistent-2               44528        35736  -19.74%
BenchmarkTCP4Persistent-3               44919        36907  -17.84%
BenchmarkTCP4PersistentTimeout          45309        33588  -25.87%
BenchmarkTCP4PersistentTimeout-2        50289        38079  -24.28%
BenchmarkTCP4PersistentTimeout-3        51559        37103  -28.04%
BenchmarkTCP6OneShot                   361305       345645   -4.33%
BenchmarkTCP6OneShot-2                 361305       331976   -8.12%
BenchmarkTCP6OneShot-3                 376929       347598   -7.78%
BenchmarkTCP6OneShotTimeout            361305       322212  -10.82%
BenchmarkTCP6OneShotTimeout-2          378882       333928  -11.86%
BenchmarkTCP6OneShotTimeout-3          388647       335881  -13.58%
BenchmarkTCP6Persistent                 47653        35345  -25.83%
BenchmarkTCP6Persistent-2               49215        35736  -27.39%
BenchmarkTCP6Persistent-3               38474        37493   -2.55%
BenchmarkTCP6PersistentTimeout          56637        34369  -39.32%
BenchmarkTCP6PersistentTimeout-2        42575        38079  -10.56%
BenchmarkTCP6PersistentTimeout-3        44137        37689  -14.61%

R=dvyukov
CC=golang-dev
https://golang.org/cl/8670044