// to, ClientConn.RoundTrip can be called with a bogus
	// new(http.Request) to release the stream reservation.
	GetClientConn(req *Request, addr string) (*http2ClientConn, error)
	MarkDead(*http2ClientConn)
}

// clientConnPoolIdleCloser is the interface implemented by ClientConnPool
// implementations which can close their idle connections.
type http2clientConnPoolIdleCloser interface {