Adds Cache Groups concepts to Cripts

[zwoop]

The goal here is to provide some cache management functionality to Cripts, allowing for a prototype implementation of the cache-groups RFC draft. In addition,t this

• cleans up the notion around cached URLs and headers, and cache keys.
• adds APIs to set the lookup status as well

The implementation adds a Cache::Groups object type to Cripts, which deals with all the details around lookups and inserts. In addition, it assures that the meta data is persistent across restarts and crashes. This is achieved with a combination of periodic disk syncs, and transaction logs. Upon startup, the synced files are loaded, following by replaying the transaction log (of any).