Use Moneta as entity and metastore

This patch replaces the entire storage system with Moneta which simplifies things a lot. Soon there will be a new Moneta release. I think rack-cache should concentrate on caching and since there are many open issues I think this is the way to go.

This would also solve the issues #67 and #49.

This patch needs more cleanup, it is just a quick hack and a proof of concept. Are you interested?