[BZ 1049054] refactor metrics migration test

There was a big in MigrateAggregateMetricsTest that I introduced in a previous
commit that actually caused the migrtion step to get skipped. I took the
opporuntity to do some refactoring to make it a little easier to write schema
update tests. I have introduced UpdateFolderFactory which is repsonsible for
creating the UpdateFolder objects. UpdateFolderFactory can be subclassed such
that tests have full control over what steps are actually executed. I have also
added some minimal verification to MigrateAggregateMetricsTest.
[BZ 1049054] fix baseline tests and clean up baseline calculation code

Some of the baseline calculation code in MetricsBaselineCalculator needed to
be updated due to my recent changes. The schema changes allowed the code to be
simplified some as well.
[BZ 1049054] clean up/remove commented out, obsolete code