This fixes a spurious "single" mode bug which occurs when using "play"
or "seek" to start playback on the song that is currently paused: in
that case, the main thread never queues the next song, and at the end
of the song, the player thread exits Run(), stopping playback, and
after that, the main thread starts the next song without considering
"single" mode.

By calling OnPlayerSync(), we ensure that the main thread gets a
chance to queue the next song before the player thread exits the Run()
loop.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/850