race_condition fix in invalidate_from_dict
fix extra loop
recompute conjunctions moved to while loop, more comments
more comments!
Code review fixes, delete RuntimeError and limit loop for 2 tries.
Merge pull request #27 from tumb1er/invalidation

Watch-free invalidation

Avoiding optimistic locking with WATCH in invalidation procedure in order to
exclude possibility of too much retiteration of transaction loop causing high
redis load.