net: band-aid for windows network poller
Fixes performance of the current windows network poller
with the new scheduler.
Gives runtime a hint when GetQueuedCompletionStatus() will block.
Fixes issue 5068.

benchmark                    old ns/op    new ns/op    delta
BenchmarkTCP4Persistent        4004000        33906  -99.15%
BenchmarkTCP4Persistent-2        21790        17513  -19.63%
BenchmarkTCP4Persistent-4        44760        34270  -23.44%
BenchmarkTCP4Persistent-6        45280        43000   -5.04%

R=golang-dev, alex.brainman, coocood, rsc
CC=golang-dev
https://codereview.appspot.com/7612045