Windows flavor of non-blocking IO, overlapped IO, differs from that on
Linux. On Windows, the OS handles writing to overlapped buffer, while
on Linux user must do it manually.
Additionally, we can only have overlapped sockets because it is the
requirement to be able to wait on them - WSAEventSelect automatically
sets socket to nonblocking mode.
So we end up emulating Linux-nonblocking sockets with
Windows-nonblocking sockets.
Pending IO state (ERROR_IO_PENDING) must not escape read/write
functions. If that happens, all synchronization like WSAPoll and
WaitForMultipleObjects stops working (WaitForMultipleObjects stops
working because with overlapped IO you are supposed to wait on an event
in OVERLAPPED structure, while we are waiting on WSA Event, see
EventLoopImplementationWindows.cpp).
