Draft: process_queue_pool: Prevent queue starvation

In process_queue_pool(), instead of processing the queue until it's empty, we collect the entries from the queue once and then process only those entries. This prevents queues from starving other queues. Without this commit, process_queue_pool() might never return, preventing all other queues from making progress: e.g., the REQ queue might prevent the HIPRIO queue from making progress.