General improvements to database backend...
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