MEDIUM: pools: centralize cache eviction in a common function

We currently have two functions to evict cold objects from local caches:
pool_evict_from_local_cache() to evict from a single cache, and
pool_evict_from_local_caches() to evict oldest objects from all caches.

The new function pool_evict_last_items() focuses on scanning oldest
objects from a pool and releasing a predefined number of them, either
to the shared pool or to the system. For now they're evicted one at a
time, but the next step will consist in creating clusters.
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