remove seenAfter offset
remove seenAfter offset

- the lastUpdate timestamp of items in the index are now updated
  everytime a new point is seen. So we dont need to add an offset
  to the lastSeen param passed to metricInde.Find().

raintank/metrictank,replay/metrictank,raintank/raintank-metric,replay/metrictank,replay/metrictank,raintank/metrictank,raintank/metrictank,raintank/metrictank,raintank/raintank-metric,replay/metrictank,raintank/raintank-metric