Implement `process::io::read/write` using Thread Pool API

process::io::read/write currently use a read/write loop with io:: poll, which is implemented by the libevent loop. This doesn't work on Windows, because libevent only works on sockets. Furthermore, the reads/writes are blocking on Windows for pipes. Instead, we should use I/O completion ports with overlapped I/O and the thread pool API to implement read/write.

Note: when read/write receive a file/socket/pipe handle in overlapped mode, they will assign them to the IOCP handle. The stout function os::nonblock will (continue to) be a no-op on Windows. It does not make sense to make os::nonblock do the association, as the IOCP handle is a global in libprocess, of which stout has no knowledge.