utils: Introduce general-purpose LRU
utils: Introduce general-purpose LRU

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.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla