When I rescan my entire library, a lot of CPU cycles are consumed for tens of
minutes.  But I don't really want to rescan everything, so I don't need all that
effort wasted; often I just want to scan one directory or file.

One common use case is the purchase of a new CD: I rip it, encode it and file it
in its proper directory.  At this point I really only want to tell SlimServer
that there's a new directory inside the main repository.

Another use case is metadata correction: today I spotted a typo in a song title
which necessitated an update of tags and a file-renaming.  I wanted to tell
SlimServer that the tags had been updated, and that the name had changed (or
that it had been removed and another added).