chore: Let db adapter to decide how to handle database target and source names.

[nashby]

When I started looking into adding sqlite support I noticed that for sqlite in database.yml you have db like db/development.sqlite3 so just adding a suffix won't work, it should have a bit different logic. So I've extracted these db name method to db adapter class and future sqlite adapter can have specific logic for it.

[theodorton]

This shows I don't have tests for the Clean command, which also uses PGCluster and this change will break that command.

PGCluster (and any future database adapters) shouldn't be aware of source or target database at the instance level - the database names should be passed as method arguments.

[theodorton]

@nashby I've added tests and took the liberty of rebasing.

[theodorton]

Aaand it doesn't break 😄 I'm still a bit hesitant about this - would like to see how it would work once there is a SQLite adapter.

[theodorton]

@nashby Most, if not all, of the necessary abstractions should now be covered by #11.