Implement this as a wrapper around other datastores

[whyrusleeping]

It would be nice to have this package be a little more modular, giving us the ability to do one or more of the following things:

• Use another datastore as the backer for the LRU (disk backed cache?)
• Use this datastore as an in memory cache for another datastore (L1, L2 style caching)

Right now this package isnt super useful outside of wanting a purely in memory lru datastore