Cleanup

[RobThree]

This commit is mostly a cleanup and refactor with these notable changes

• It puts RDBMS specific stuff in their respective 'repositories' (by lack of a better name) and has the ProcessManagersStore and OutboxBackend ask for interfaces of these repositores.
• The integrationtests have been cleaned up and combined where possible to be more DRY
• Files with more than 1 class or interface are (with few exceptions) given their own file

Furthermore, minor formatting issues have been fixed or reverted. I have 'squashed' all commits that had some rebase-weirdness by the age-old tried and tested method of zipping the project, deleting the project, cloning and overwriting the changes from the zip 😬

There are no functional changes.

This should be my last PR (for now 😇 )

[AlexeyRaga]

Thanks!