RawRead assumes the callback will perform either (a) a blocking read
and always return true, (b) a blocking read with a SO_RCVTIMEO set
returning false on WSAETIMEDOUT, or (c) a non-blocking read
returning false on WSAEWOULDBLOCK. In the latter two cases, it uses
a 0-byte overlapped read for notifications from the IOCP runtime
when the socket becomes readable before trying again.

RawWrite assumes the callback will perform blocking write and will
always return true, and makes no effort to tie into the runtime loop.

Change-Id: Ib10074e9d502c040294f41a260e561e84208652f
Reviewed-on: https://go-review.googlesource.com/76391
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>