MPDStatistics: Return null from dbUpdate() if not yet refreshed.
dmix: Repair regressions from 9201c70.

I didn't test commit 9201c70 well (read: at all). This fix retains many of the
ideas from commit 9201c70, while reverting a few.
MPDAsyncHelper: Use album cache by setting.

A regression, most likely from 02c6e3f.
MPD.java: Crop only if the playlist length is greater than 1.
MPDPlaylist: Use status changes to refresh for playlist changes.

Idle playlist subsystem is not the optimal place to check for playlist
updates, it can actually cause huge inconsistencies (as I learned earlier
in 55029f9).