Complete overhaul of JSON metadata exchange code. Removed global method getArtistsAsJSON() and Song, Artist, Album toJSON() methods, replacing them with a DatabaseHandler method, getLibraryAsJSON(), which pulls from the database rather than global data structures. Also rewrote loadMusicFromJSON() in JoinJamThread and changed it to add to DB instead of globals. Streamlined MusicLibraryLoader code and removed most global data structures.