This PR changes to use relative database paths by default. The default mode is to store all databases in the same folder.
With this update, the paths stored in the server database can now be relative, in which case they are resolved relative to the datafolder.
This makes it simpler to move the data folder as the paths are not stored in full.
For new backups, relative paths are assigned.
For existing backups, the full paths are retained.
If the database path is updated manually, the path will be made relative, if it is relative to the datafolder; otherwise a full path is stored.
This fixes#6677
This PR has a large blast radius because it takes the final step and bumps up the Controller to be fully async.
We have historically done a piece-by-piece update, so all operations were already async but the controller interface was kept synchronous.
With this update, the controller is now fully async and all tests are updated.
Most places where the new C# compiler warns about function names not ending in `Async` were also adressed, giving a massive refactor change.
Functionally, no changes are done.
Kenneth Skovhede