Scan for catalog changes on startup

[shane-mason]

Read directories on startup and update any changed entries. This should be the default mode -

--skip_scan - bypass change scan and just generate a schedule and exit --rebuild_catalog - keep as-is to force full rebuilds

For initial implementation, the scan will determine change if it detects any movie files that have an updated timestamp after the catalogs generated timestamp. Will trigger a full rebuild rather than an incremental update.