close: make close notification go through writable

close: make close notification go through writable

Until now we took the approach if just writing the close notification
broke something, we didn't care because we were closing the connection
anyway.

But with lws_meta, breaking stuff in the parent connection would be a
sticky problem outliving the closing child connection.

So this adds a new wsi state LWSS_WAITING_TO_SEND_CLOSE_NOTIFICATION
and makes the send go via the writable callback mechanism.