Use Guzzle delay option instead of blocking via usleep

[ostrolucky]

Guzzle provides delay option https://docs.guzzlephp.org/en/stable/request-options.html#delay which can be used to delay sending of the request. CurlMultiHandler utilises it in non-blocking manner. It would be great if this middleware used it instead of calling usleep()

[freekmurze]

Feel free to send a PR for this. All current test should remain green.