This found a small number of real bugs too, for example,
this one that looked weird because of a 2to3 conversion,
but was wrong both before and after:
-        except IndexError as TypeError:
+        except (IndexError, TypeError):

Also fix a bug when reporting bad units on "keep" configuration key

This is instead of hardcoding 500

Might fix the problem Mark noticed where turn-off transients
are erroneously matching the drop that follows startup transients.

If the max scale in the exemplar was a column we weren't using, it
would bail out when looking for that correlation later.  Change things
around so exemplars in RAM only keep around the columns we care about.