ENG-4428: Use VoltTableUtil.unionTable() for fragment result aggregation in all
sysprocs.
Remove unused accessor to expose leader appointer to the world in RealVoltDB per
Ryan's review comment.
Move a method into base class RejoinCoordinator per Ryan's comment.
Remove unnecessary type castings from RealVoltDB.initialize() per Ryan's comment.
Move testUnionTables to TestVoltTableUtil.
Merge pull request #414 from VoltDB/ENG-4428

ENG-4428: use VoltTableUtil.unionTable() for aggregations in sysprocs.