Heavily refactoring DruidNodeDruidQueryExecutor to support both POST and GET
Improving DruidNodeDruidQueryExecutor's unit tests
Removing unneeded class
Updating TimeBoundaryDruidQueryGenerator to return a string and not an assoc array
Updating SegmentMetadataDruidQueryGenerator to return a string and not an assoc array
Renamed TestGroupByDruidQueryGenerator to SimpleGroupByDruidQueryGenerator
Applied the Interval class to it