Simplify migration code
Description
We need to have a more organized and simpler to implement migration mechanism - this part was not really done properly since unity 2 release.
Things to consider:
a single interface to be implemented covering all migration paths
zero wiring of migrations: bean implementing interface is implemented, app schema version increased and that's all.
single number versioning of schema revisions.
common versioning of JSON dumps and SQL schema dumps, with empty migrations if only one of those is actually touched.
use of flyway/liquibase for schema migrations
