Make it possible to call key functions when extending Dataloader

[alafon]

We might need to call getCacheKeyFromKey and checkKey when extending the Dataloader class, in order to have homogeneous keys.

Here is an exemple:

    public function load($key)
    {
        $cacheItem = $this->primeCache->getItem($this->getCacheKeyFromKey($key));

        if (!$cacheItem->isHit()) {
            $promise = parent::load($key);
            if ($promise instanceof Promise) {
                $promise->then(function ($value) use ($key) {
                    $this->prime($key, $value);
                });
            }
            return $promise;
        } else {
            return $cacheItem->get();
        }
    }