Add uv_try_write on windows

> I think it should be returning some sort of EAGAIN if WSAGetLastError() is WSAEWOULDBLOCK.

Does uv_translate_sys_error not do this?

> Not only, we also need to return it if there are other write requests pending. That's why I we need to wait for #1439.

It would be possible to just look at the `write_reqs_pending` field and check if there are pending overlapped requests. That seems like a simple addition to this patch which would make it correct.

https://github.com/joyent/libuv/blob/49cb40c47902dd04c7787e6543e9e13368dc1840/include/uv-win.h#L412