Terminate all MPV instances when stopping Navidrome (#3008)

* Terminate all mpv instances when stopping Navidrome

* Exit trackSwitcher goroutine when terminating

* Remove potential race condition when starting the Playback device

* Fix lint error

* Removed unused and unneeded vars/functions

* Use device short name in log

* Small refactor

* Small nitpick

* Make start functions more uniform

core/playback/queue.go

@@ -134,17 +134,3 @@ func (pd *Queue) IncreaseIndex() {
 		pd.SetIndex(pd.Index + 1)
 	}
 }
-
-func max(x, y int) int {
-	if x < y {
-		return y
-	}
-	return x
-}
-
-func min(x, y int) int {
-	if x > y {
-		return y
-	}
-	return x
-}