Here, i've made readDirectoryDB detect when stuff is deleted, added and updated.
So after a update, and we call redDirectoryDB, we update the db instead of
just adding stuff w/o "updating" and "deleting" stuff as neccessary.

git-svn-id: https://svn.musicpd.org/mpd/trunk@659 09075e82-0dd4-0310-85a5-a0d7c8717e4f