vorbis comments are updated on the fly for streams
need to decode icy metadata
buffering of metadata needs to be hardened, to ensure that player has already read a particular metachunk or passed over it
git-svn-id: https://svn.musicpd.org/mpd/trunk@1358 09075e82-0dd4-0310-85a5-a0d7c8717e4f
