VoltSystemProcedure: Add more utility methods
VoltSystemProcedure: Add more utility methods

Add utility methods for creating and executing plan fragments. The common use
case is to create the two plan fragments and then execute them. Bundle that
into one utility method. Also, add a constructor to SynthesizedPlanFragment to
simplify initialization.

VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb