[RF] Deallocate memory owned by empty MemPoolForRooSets Arenas
[RF] Deallocate memory owned by empty MemPoolForRooSets Arenas

If an arena in the MemPoolForRooSets is not referenced anymore, it
should delete the memory it has allocated.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root