Improve scalability/performance of daily scan

Migrated from: https://tracker.clearos.com/view.php?id=12851

Reported by: Ben Chambers

Waste of CPU/cycles to do a daily scan on files that haven't changed in months...especially if server has over 6M files.

Improve logic of scan so that it only checks files that have changed since last scan (after initial full scan has been completed) by combining clamscan with find -type f -mtime -1 etc.