A Cache is an interface that maps keys to values. It has internal synchronization and may be safely accessed concurrently from multiple threads. It may automatically evict entries to make room for new entries. Values have a specified charge against the cache capacity. For example, a cache where the values are variable length strings, may use the length of the string as the charge for the string. A builtin cache implementation with a least-recently-used eviction policy is provided. Clients may use their own implementations if they want something more sophisticated (like scan-resistance, a custom eviction policy, variable cache sizing, etc.)
