This changes uv-win to use the new uv_req subclasses.
It gets rid of the uv_req.flags field. There used to be only request flag
(UV_REQ_PENDING), and it was mostly obsolete; it only had a real purpose for
internal uv_read requests. Now we'll use the UV_HANDLE_READ_PENDING flag on
the handle instead.
This patch also simplifies the accept logic for named pipes on windows. We
no longer have a separate struct to store information about established
connections. Instead we just carry over the windows HANDLE from the accept
request to the client handle in uv_pipe_accept().
Instead of uv_shutdown, uv_write, uv_connect taking raw uv_req_t we subclass
uv_req_t into uv_shutdown_t, uv_write_t, and uv_connect_t.
uv_req_init is removed.
* Renamed test_connection_fail to connection_fail following the conventions.
* Mapped Windows error ERROR_CONNECTION_REFUSED to OIO_ECONNREFUSED.
* Asserting that the oio_err in connection_fail is OIO_ECONNREFUSED.
