Make "parse_reports" task use schema API (remove Amazon SimpleDB)

[preaction]

Once the new API for the local metabase cache is working (https://github.com/cpan-testers/cpantesters-schema/issues/4), the ParseReports task should be edited to use it:

• Use CPAN::Testers::Schema::ResultSet::Metabase to get unprocessed raw reports
  • Rather than querying the SimpleDB for all the time periods, we should need only to look for reports that do not have an associated stats row (have the id field set to NULL).
• Use the as_metabase_fact method to get the deserialized Metabase facts
• Use CPAN::Testers::Schema::ResultSet::Stats to insert the new statistics rows
• Update the metabase cache with the ID of the new stats row to mark it as completed

This should be done after #1.

[preaction]

Since we could not create a schema API onto the Metabase cache, we can not do things this way. Instead, updating the Metabase cache is part of #1.