Cannot use WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE.

In copy-out mode, the frontend should not send any messages until the
backend has finished streaming, by sending a CopyDone message. I'm not sure
if it would be legal for the client to send a new query before receiving the
CopyDone message from the backend, but trying to support that would require
bigger changes to the backend code structure.

Fixes an assertion failure reported by Fujii Masao.