[4/X] Update feature/sql-refractor | Migrator concept & legacy migration

[MarkL4YG]

This pull request includes:

• An unfinished MigrationManager which already has some methods it is going to need in the future.
• An abstract Migrator class from which all migrators will inherit.
• An abstract DBMigrator class from which all db migrators will inherit which provides many utility methods for working with the tables/schema.
• An first try on a LegacyDBMigrator which will migrate the existing database schema to the new one.

Of course, since this is yet another update of the big thing, this code is not yet runnable.

The basic idea behind migrators

The MigrationManager determines which migrations to run based on the current schema version and whether or not the schema has already been initialized. This can be used for both the file and DB storage.
Migrators then - well - migrate the data to the latest version from whatever version currently is used.

Notes

• The migration must be stable which is why the TEMigrationException will serve an important role in future code updates (e.g. triggering rollbacks when things go wrong).
• The schema has been slightly updated as moving jobs to UUIDs is easier in a seperate update than the migration from legacy to latest.
• I definitely want to add more Javadoc but that's going to have to wait until I have the migration process completely laid out including the manager

[MarkL4YG]

Note to self: Rebase the sql-refractor onto develop branch after this has been merged and open another PR for that.

[MarkL4YG]

As suggested in private conversations, I'll be closing this PR. It will be replaced with a MVP implementation of the refactor.