union plan node memory leak
bug fixes
Made union executors stateless - no need for reset
Merge remote-tracking branch 'alexeev/eng_3217' into ENG-3217
sqlcoverage re-enable parenthesized union tests and fix some "suddenly?" non-deterministic cases
Merge branch 'master' into ENG-3217
Merge remote-tracking branch 'origin/master' into ENG-3217
Merge branch 'master' into ENG-3217
Merge pull request #335 from VoltDB/ENG-3217

Eng 3217 - fix issues with EXCEPT and other nested set ops, including memcheck failures