Run descriptor parser over all prior descriptors

Stem's descriptor parser has gotten a pretty good workout, being exercised every time we do a run of the integ tests. However, I've only done spot checks for historical data.

We should talk with Karsten about running a small stem script on one of the metrics hosts that attempts to parse all of the historical descriptors. The script would be trivial to write, and given a week or so we'd know either that stem can handle all historical descriptor content, or where the issues lay.