Optimize scan to only instrument new files

[dresselm]

The scanner currently walks the entire directory structure and writes out metadata files within each directory. Running scan again will do the same. There are several easy scanning optimizations:

• Instead of writing over the metadata file, amend it
• Touch the metadata file after each run, even if it's not modified, so that we can compare it's modified time to the media in the directory. If any media have a more recent modified time, re-instrument them.