Revert "client: Make sure fd is writable before calling send() to avoid blocking"

This reverts commit 23ecea077d8f22d9da5cae50df3e2ff3406fee90.
Use non-blocking sockets for client communication

This approach is better than using blocking sockets and select() since
there's no guarantee that send() doesn't block. Plus we're using poll()
anyway so send() and recv() will only be called if the socket is actually
ready for writing/reading.