Update _beet; incremental caching
Update _beet; incremental caching

Lots of changes to implement incremental caching scheme.
shamangeorge/beets,beetbox/beets,SusannaMaria/beets,sampsyo/beets,shamangeorge/beets,shamangeorge/beets,jackwilsdon/beets,ibmibmibm/beets,sampsyo/beets,jackwilsdon/beets,ibmibmibm/beets,sampsyo/beets,jackwilsdon/beets,sampsyo/beets,beetbox/beets,SusannaMaria/beets,SusannaMaria/beets,beetbox/beets,shamangeorge/beets,ibmibmibm/beets,ibmibmibm/beets,jackwilsdon/beets,beetbox/beets,SusannaMaria/beets