net: use SetFileCompletionNotificationModes on windows if available
This allows to skip GetQueuedCompletionStatus if an IO operation completes synchronously.
benchmark                    old ns/op    new ns/op    delta
BenchmarkTCP4Persistent          27669        25863   -6.53%
BenchmarkTCP4Persistent-2        18173        15908  -12.46%
BenchmarkTCP4Persistent-4        10390         9766   -6.01%

R=golang-dev, mikioh.mikioh, alex.brainman
CC=golang-dev
https://codereview.appspot.com/12409044