Fix diskFrontier.ContainsFingerprint() return value.
Fix nil pointer exception in frontier building.
Fix iterator behavior in view.GetSampleAtTime()
Rename extractSampleValue -> extractSampleValues.
Fix edge cases in data extraction for point and interval ops.
Replace renderView() by cleaner and more correct reimplementation.
Merge branch 'refactor/storage/batching' of github.com:/prometheus/prometheus into refactor/storage/batching

Conflicts:
	storage/metric/operation.go
Adopt new ops sorting interface in view rendering.