audioscrobbler: rewrite for new protocol 1.2

allocate as much as we can on the stack, to avoid numerous malloc() & free()
fix memory leaks by the same way
metadata reading is now performed exclusively from the callbacks
don't use linked lists anymore, but a queue of a fixed size of 50 songs, i.e. the maximum we can transmit in one submission.
properly unloads the module when encountering a fatal error, like no username/pw, no memory, VLC being banned by last.fm servers ...