Add initial support for feed-of-feeds in FeedArchivePastToPresentReader
Refactor EntryRdfValidatorHandler and make more methods public
Move down binding of ?datatype in expected_datatype.rq to prevent possible bug in Sesame
Implement feed-of-feeds update tracking

- Rename CompleteFeedEntryIdIndex to FeedEntryDataIndex and change from IRI Set to IRI+AtomDate Map.
- Use FeedEntryDataIndex to store feed-of-feeds data to enable tracking of nested feed updates.
Merge branch 'master' into feature/collect-feed-of-feeds
Factor out FeedEntryDataIndexMemImpl from CompleteFeedReaderSpec
Add FeedOfFeedsReaderSpec and fix bugs in feed-of-feeds collecting
Change some TODOs to IMPROVE notes in FeedArchivePastToPresentReader
Merge branch 'feature/collect-feed-of-feeds' into develop