Need a way to clear old states

Currently once something is recorded in the state database it's there forever. An example of when this is bad:
- You launch an instance and configure it to be monitored in NYMMS
- Eventually you replace that instance with a new instance and update your NYMMS config. Now you're monitoring the new instance, but not the old.

In that case the old state will remain in the state backend, even though it won't be used for anything. It might be worth having the scheduler or something similar clean up old states - the only issue with that is that we'd have to find some way to deal with passive monitors since the scheduler has no concept of passive monitors (the passive monitors just send in results themselves). That may be a bug in and of itself.