CLEANUP: pools: make the local cache allocator fall back to the shared cache

CLEANUP: pools: make the local cache allocator fall back to the shared cache

Now when pool_get_from_local_cache() fails, it automatically falls back
to pool_get_from_shared_cache(), which used to always be done in
pool_get_from_cache(). Thus now the API is simpler as we always allocate
and free from/to the local caches.