Backport of Fix TopN with BySegment=true (#845) for 0.6.x
See https://github.com/metamx/druid/pull/856 for more details
Does not include unit tests
Remove getIntervalString from BySegmentResultValue
Merge pull request #863 from metamx/0.6.x-fixFor-845-rebase

Backport of Fix TopN with BySegment=true (#845) for 0.6.x