Replace MenuFactory SplPriorityQueue by an array.

Weird ... but it seems to fix our issue.

We were running out of memory in our functional tests, after updating to the commit https://github.com/KnpLabs/KnpMenuBundle/commit/fe44ad0cba7dff6a75d303816f846c93f49a5658 mentioned in https://github.com/KnpLabs/KnpMenuBundle/issues/198 .