Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779d).
This fixes a deadlock bug introduced by 18076ac9. After all, the
second Cond was necessary.
The problem: two threads can wait for a signal at the same time. The
player thread waits for the output thread to finish playback. The
main thread waits for the player thread to complete a command. The
output thread finishes playback, and sends a signal, which
unfortunately does not wake up the player thread, but the main
thread. The main thread sees that the command is still not finished,
and waits again. The signal is lost forever, and MPD is deadlocked.
