MEDIUM: pools: unify pool_refill_alloc() across all models

MEDIUM: pools: unify pool_refill_alloc() across all models

Now we don't loop anymore trying to refill multiple items at once, and
an allocated object is directly returned to the requester instead of
being stored into the shared pool. This has multiple benefits. The
first one is that no locking is needed anymore on the allocation path
and the second one is that the loop will no longer cause latency spikes.