We only need to lock sv->nr changes to prevent traversals ( why
it's called "nr_lock"). free(3) is a "slow" function on my
system; so we can avoid unnecessarily holding a lock long for
longer than needed.
Only one lock is used for all songvec traversals since
they're rarely changed. Also, minimize lock time and
release it before calling iterator functions since they
may block (updateSongInfo => stat/open/seek/read).
This lock only protects songvecs (and all of them) during
traversals; not the individual song structures themselves.
Our linked-list implementation is wasteful and the
SongList isn't modified enough to benefit from being a linked
list. So use a more compact array of song pointers which
saves ~200K on a library with ~9K songs (on x86-32).
