storage: add deduplication function
storage: add deduplication function

This adds a function to deduplicate two series sets given that duplicate
series have equivalent data points.

alileza/prometheus,GoogleCloudPlatform/prometheus,Conorbro/prometheus,alileza/prometheus,Conorbro/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,kangwoo/prometheus,amorken/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,AlekSi/prometheus,yanghongkjxy/prometheus,prometheus/prometheus,keegancsmith/prometheus,brancz/prometheus,kangwoo/prometheus,Stackdriver/stackdriver-prometheus,alileza/prometheus,yanghongkjxy/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,amorken/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,Conorbro/prometheus,brancz/prometheus,amorken/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,Stackdriver/stackdriver-prometheus,Stackdriver/stackdriver-prometheus,amorken/prometheus,AlekSi/prometheus,yanghongkjxy/prometheus,brancz/prometheus,AlekSi/prometheus,yanghongkjxy/prometheus,GoogleCloudPlatform/prometheus,Stackdriver/stackdriver-prometheus,Conorbro/prometheus,alileza/prometheus,alileza/prometheus,AlekSi/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,keegancsmith/prometheus,brancz/prometheus,keegancsmith/prometheus