MEDIUM: pool: compute the number of evictable entries once per pool

MEDIUM: pool: compute the number of evictable entries once per pool

In pool_evict_from_local_cache() we used to check for room left in the
pool for each and every object. Now we compute the value before entering
the loop and keep into a local list what has to be released, and call
the OS-specific functions for the other ones.

It should already save some cycles since it's not needed anymore to
recheck for the pool's filling status. But the main expected benefit
comes from the ability to pre-construct a list of all releasable
objects, that will later help with grouping them.