fix proxy server memory leak

[ShangSky]

// pkg/proxy/proxy.go
err = http.Serve(l, p)
if err != nil && !errors.Is(err, ErrAlreadyAccepted) {
p.logger.Errorw("Serving HTTP request failed.",
	"error", err)
}

<-clientConnNotify.closed // will block

// pkg/proxy/net.go
func (c *ConnNotify) Close() {
	c.Conn.Close()
	c.closed <- struct{}{} // will block
}