PersistentObjectCache should be moved to DirectWriteBlueprintsStore

[gdaniel]

This is the case for other backend implementations.

Need to be careful: Blueprints Store internal cache is used in other projects (Mogwai, PrefetchML). Modifying it may cause unexpected behavior / performance losses in this tools.

[yvrng]

Fixed with #69 : This cache is now common for all implementations