MEDIUM: pools: start to batch eviction from local caches

MEDIUM: pools: start to batch eviction from local caches

Since previous patch we can forcefully evict multiple objects from the
local cache, even when evicting basd on the LRU entries. Let's define
a compile-time configurable setting to batch releasing of objects. For
now we set this value to 8 items per round.

This is marked medium because eviction from the LRU will slightly change
in order to group the last items that are freed within a single cache
instead of accurately scanning only the oldest ones exactly in their
order of appearance. But this is required in order to evolve towards
batched removals.