grab all free objects in a folio

[kuszmaul]

When we go to the global data structure to get objects, we should go ahead and fetch all the free objects out of a folio, since it's the same amount of lock contention. We don't actually expect to get lots of free objects out of a folio on average for random malloc/free workloads, but there are workloads where this optimization helps, and I don't see how it can hurt.