On windows, initialize IO types (for example, sockets) with WSA_FLAG_OVERLAPPED

[carllerche]

As per the docs: https://msdn.microsoft.com/en-us/library/windows/desktop/ms742212%28v=vs.85%29.aspx

Most sockets should be created with this flag set. Overlapped sockets can utilize WSASend, WSASendTo, WSARecv, WSARecvFrom, and WSAIoctl for overlapped I/O operations, which allow multiple operations to be initiated and in progress simultaneously.

If I understand correctly, this flag must be set at initialization time of the socket (and other types), does not have a cost, and enables the socket to be used in overlap mode w/ completion ports.

Doing this will be helpful for when I add windows support to mio.

cc @alexcrichton