[BUG FIX] for https://github.com/ruohoruotsi/DPMusicController/issues/3 Successive clicks to the Previous/Next Track button, causes audio engine to freak out (constant re-initing & configuring) then crashes. The issue is there is a timer with fires repeatedly over the course of 6 seconds to ramp down/up for the crossfade and if during that time another button hit event comes in from next/previousWithCrossfade, it cleans up objects being actively crossfaded. This code is not thread-safe and I'll be completely overhauling crossfade shortly. In the meantime, we don't crossfade, just play sequentially.