General improvements to database backend DB backend changes: * add db.get_history method * db.find_records returns list, allows filtering of keys * add db_backend tests New Client methods exposing db backends: * rc.hub_history() # proxy for db.get_history * rc.db_query() # proxy for db.find_records Extra fixes along the way: * Harder for db issues to crash the Hub * Better JSON handling of date objects * Don't raise NoEnginesRegistered when load-balancing on all engines * rc.get_result works with whole history if database is persistent closes gh-391