Hostdb scanner

This does 3 things:

1. Makes sure that when the hosts get scanned which are both online and offline
2. compresses the scan histories, so only the most recent 30 days of scans are kept. this I think will amount to between 20kb and 50kb per host, so still a pretty significant footprint. But at least somewhat bounded.
3. toss out hosts that have been offline for more than a month so that they aren't cluttering the json files and slowing everything down.