Sharding exception on querying for old data

[sharnik]

As is: When you query an existing timeseries for some old period when there's no data, it throws a "Couldn't look up columns" error. My initial look suggests that it's due to not finding the proper shard to host this data, as it fails here: https://github.com/influxdb/influxdb/blob/master/coordinator/coordinator.go#L272-L279

As should be: should return 0 results.

Here I added an extra assertion to an existing test to see how it fails: sharnik@2863f8e