feat: faster loading for collection cache

[r-o-b-o-t-o]

Sped up the initial collection cache loading: it used to take ~75 seconds for about 20 million rows on my system, now takes ~15 seconds.
The slowdown was caused by the reordering of the vector at each insert (no idea why it was even being sorted in the first place, it's only item GUIDs, which have no relevance to the EnableSortByQualityAndName setting...)
The change from vector to unordered_set isn't really that impactful performance-wise but it helps make the code a bit cleaner in my opinion.

[Helias]

Thanks!
you could also improve

this

        uint32 accountId = player->GetSession()->GetAccountId();
        if (sT->collectionCache.find(accountId) == sT->collectionCache.end())
            return allowedItems;

instead of using .find() we can use .contains now, right?