Short-cut user-user neighbor finding

- Use shorter of user items or target items to find candidate neighbors
- Check that user similarity is sparse
Implement item event collections
Implement grouping cursor base class
Add streamEventsByItem
Use item-event streaming for itemwise item-item build
Merge pull request #493 from elehack/feature/elehack-item-stream

Implement item-based event streaming.
Add filtered streaming (#492)
Add #492 to release notes