GetQueuedCompletionStatusEx allows to dequeue a batch of completion
notifications, which is more efficient than dequeueing one by one.

benchmark                           old ns/op    new ns/op    delta
BenchmarkClientServerParallel4         100605        90945   -9.60%
BenchmarkClientServerParallel4-2        90225        74504  -17.42%

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/12436044