Add filesystem storage option for periodic checks

[pjreed]

The FilesystemBagStorage implementation currently relies on getting inotify events to know when the filesystem has changed and update the database, but inotify events may not work for some filesystems, such as CIFS.

It'd be useful to have an option in the config to periodically check the filesystem for changes, and maybe even to detect whether inotify events are available and automatically enable periodic scans if they're not.