* Fix: apply drop checkConstraints properly which were not applied for Enum to String
* Use IF EXISTS to drop constraints
* added TODO and comment about MariaDB supporting CHECK constraints from version 10.2.1 forward compared to MySQL 5.7/8.0 parsing but ignoring CHECK constraints
* updated reference scripts for dropping constraints on enums
* trigger based history will recreate view properly and handle default value also for history view
* HistoryTriggers are generated at the end
* Reference Script
* MYSQL: lock all tables at once
* NEW: Ebean reads version from pop.properties and prints it at start up
* DDL generation and migration can append a header and platformMigration is prepared to append epilog & prolog to scripts
* FIX: Set identity only, if it is not the platform default type
* Updated reference scripts
* ADD basic support for foreign key migration
* DbMigration detects alter foreign keys
* tests
