DB migration refactoring.

1. check if the current migration mechanism is able to migrate a version 1.0 DB up to latest master
2. measure how much time it takes on large datasets
3. refactor the migration mechanism:
  3.1 move from dicotomic approach "latest schema OR migrations from old db" to "original schema + migrations"
  3.2 store all versions (migration date, migration script name, ...) information in the versioning table instead of the latest
  3.3 all .sql files in the same step directory must by applied atomically
  3.4 zero filling directory step names for sensible sorting
4 enhance packaging upgrade script to ask to user what to do with the preview DB (rename and start from a empty , upgrade, if you want to backup your current version break the upgrade and perform it manually).
5 downgrade warning: if a db exists the user must be advise that the procedure destroy the current DB (performed by more recent package scripts)
6 implement automatic tests