Add ability to break out of idle using timeout callback

[stevebauman]

Closes #97

This PR adds the ability to break out of mailbox idle by supplying a callback into the timeout parameter.

A timeout callback is called for each iteration of the idle loop, allowing it to effectively break out of it.

• Returning false from the callback will result in the idle cancelled.
• Returning a number from the callback will result in the number being used as the timeout for the upcoming idle.

$loop = 0;

$mailbox->inbox()->idle(
    callback: function (Message $message) {
        // ...
    },
    timeout: function () use (&$loop) {
        if ($loop > 5) {
            return false; // break after 5 IDLE's
        }

        try {
            return 100; // timeout in seconds
        } finally {
            $loop++;
        }
    }
);