This commit changes a minor queue priority design to something which
makes a little bit more sense.

Previously, a song that had already been played would only be
re-enqueued if its priority had just been raised above the current
song's.  This means that if it was already above, it was not
re-enqueued.  That is a surprising behavior, because users expect a
song to be played when its priority is raised.

Now the song is always re-enqueued if its priority is raised (and
above the current song's - no matter if it has already been above
before).

 https://bugs.musicpd.org/view.php?id=4592

Failure must be indicated by returning false.

Fixes regression by commit 42f7df96

http://bugs.musicpd.org/view.php?id=4470

This is necessary to avoid an assertion failure in UpdateQueuedSong()
after LockSeek() has failed.

Fixes disappearing duration of remote songs during playback.

See http://bugs.musicpd.org/view.php?id=4492

Regression by commit 738583e3