Clean-up the upgrade path: aggregator

The whole upgrade path of Drupal 6 to Drupal 7 is in urgent need of clean-up:

• We need to remove any call to API functions
• We need to collapse table modifications into the same functions, and avoid changing the table schema several times during the upgrade process
• We need to remove unneeded upgrade functions and clean-up the numbering

Starting here with the aggregator module.