parallel-merge unit test assertions

Using serialize for merge allows less threads to be used under contention instead of blocking and using them all.
This changes the assertion to be <= 3 instead of == 3 because of that.
Merge pull request #976 from benjchristensen/parallel-merge-assertions

parallel-merge unit test assertions
Dematerialize - handle non-materialized terminal events
Merge pull request #977 from benjchristensen/dematerialize

Dematerialize - handle non-materialized terminal events
Pivot Operator
Merge pull request #982 from benjchristensen/pivot

Pivot Operator