Prevent retrieval of stale renditions for non-versionable documents

Currently, the latest rendition for a non-versionable document is understood to be the rendition with the latest dc:modified property. This definition is problematic, especially for long-running renditions which are common for non-versionable, folderish documents.

The definition of latest rendition needs to be more precise such that it correlates to the source document's sourceDocumentModificationDate.

The proposed fix is to define a new datetime rendition schema element rend:sourceModificationDate. The RenditionCreator would populate this value with the source's sourceDocumentModificationDate property value. The RenditionFinder would be modified to exclude renditions whose rend:sourceModificationDate is earlier than the source's sourceModificationDate.