Allow handle deliveries concurrently

[klimick]

Closes #80

[klimick]

After the changes in the Consumer, a race condition appeared here.

Here is a minimized race condition.

/** @var DeferredFuture<null> */
$deferred = new DeferredFuture();

/** @var \ArrayObject<int, string> */
$consumed = new \ArrayObject();

$callback = static function () use ($consumed, $deferred) {
    $consumed[] = 'elem';

    // Simulates an I/O operation (e.g., $delivery->ack())
    delay(0.1);

    // After resuming, both fibers will see that \count($consumed) === 2
    if (\count($consumed) === 2) {
        // Both fibers attempt to complete the same DeferredFuture
        // Error: the operation is no longer pending
        $deferred->complete();
    }
};

await([
    async($callback),
    async($callback),
    $deferred->getFuture(),
]);