Ensure labels are sorted and replica labels are included

Also send LabelSets in the response for newer Thanos versions (fixes #17).

This fixes 2 bugs, which combined made this a very confusing set of issues:

Not including replica labels in the store labels (i.e. per timeseries). We actually weren't getting deduplication across OpenTSDB instances. Worse (and more confusingly) because this was just combining everything into the same series if there were multiple instances which had exactly the same timeseries labels and timestamp it may overlap and generate entirely invalid chunks.
Labels were not sorted, which Thanos relies on for deduplication. Because opentsdb-goclient uses a go map to store the labels the labels actually had random ordering, so the previous issue sometimes manifested or not, depending if the sets got combined.

G-Research / geras