Implement time-to-live (TTL) eviction in DiskMemoryPersistenceStack

[p4checo]

Even though we intend to break our DiskMemoryPersistenceStack into smaller, more specialized persistence stacks/stores, we should implement a TTL mechanism on the Disk persistence, so that items can be evicted if they are older than X amount of time.

This is useful so that we don't have to just rely on the less frequently used (last accessed time) to evict stale items from the cache.