Update of fixity infrastructure in order to be able to create Fixity results for projected Datastreams

As per discussion on Friday, this branch adds a CacheEntry interface, to decouple fixity calculation from the LowLevelStoreService and LowLevelCacheEntry.
In order to be able to distinguish projected nodes from "normal" nodes the mixin type "fedora:projection" has been added and Modeshape's FileSystemConnector has been extended so that the mixin type is added on the fly to projected fedora:resources.