DEBUG: pools: let's add reverse mapping from cache heads to thread and pool

DEBUG: pools: let's add reverse mapping from cache heads to thread and pool

During global eviction we're visiting nodes from the LRU tail and we
determine their pool cache head and their pool. In order to make sure
we never mess up, let's add some backwards pointer to the thread number
and pool from the pool_cache_head. It's 64-byte aligned anyway so we're
not wasting space and it helps for debugging and will prevent memory
corruption the earliest possible.