see https://github.com/frankmcsherry/differential-dataflow/issues/113

- a new Variable implementation by Frank that reduces the cost of
recursive variables by 1/3
- use distinct_total instead of distinct where safe
- don't create recursive variables for non-recursive collections
- use 32-bit timestamp for nested scopes

- differential moved to tag-based release model
- use barrier synchronization to block worker threads instead of idling
when there is no work to do