In preparation for tracking different kinds of objects, not just
rows_entry, in the LRU, switch to the LRU implementation form
utils/lru.hh which can hold arbitrary element type.
The LRU can link objects of different types, which is achieved by
having a virtual base class called "evictable" from which the linked
objects should inherit. Whe the object is removed from the LRU,
evictable::on_evicted() is called.
The container is non-owning.
