#59: Cache für OSM-Kacheln und Hintergrund überarbeiten (http://polymap.org/atlas/ticket/59)
  - UnitOfWorkInstance: use soft references instead of Cache (see code for more details)
#145: Datenhaltung für komplexe Datentypen (http://polymap.org/polymap3/ticket/145)
  - EntityInstance: comment about NoSuchEntityException
  - EntityPropertyInstance: check aNewValue != value before setting value
  - UnitOfWorkInstance: switch back to Cache as the default impl is now Soft2Cache