Fix rollup start time inconsistency for dynamic rollups

Here is a snippet of the response we sent out:

{"numPoints":10,"average":121,"timestamp":1392255300000},{"numPoints":10,"average":120,"timestamp":1392255600000},{"numPoints":10,"average":127,"timestamp":1392255918600},{"numPoints":10,"average":126,"timestamp":1392256218602},{"numPoints":10,"average":123,"timestamp":1392256518616},{"numPoints":10,"average":117,"timestamp":1392256818599},{"numPoints":10,"average":119,"timestamp":1392257118589},{"numPoints":10,"average":118,"timestamp":1392257418598},{"numPoints":10,"average":116,"timestamp":1392257718589},{"numPoints":10,"average":115,"timestamp":1392258018602},{"numPoints":10,"average":121,"timestamp":1392258318602},{"numPoints":10,"average":123,"timestamp":1392258618622},{"numPoints":10,"average":122,"timestamp":1392258918615},{"numPoints":10,"average":119,"timestamp":1392259218591},

As you can see, the timestamps for dynamic rollups use the first timestamp of the metric in the slot instead of the slot boundary. This PR fixes it.