Implement Cleaners

[schrockn]

Eventual consistency in Kvetch is guaranteed by "cleaners" which ensure that indexes and edges correctly mirror the canonical data stored on objects. These are not yet implemented. This is a strict prerequisite for deploying this system to production. The cleaner would periodically "wake up" and query the tables to see if any objects have been updated and then also querying the index (or edge table) to see if that has been updated. This covers the case where an object is persisted and then before the corresponding index and edge entries are persisted the process dies or some other error occurs. These cleaners are the price you pay for no cross-node transactional infrastructure.