Replace `*_for_db` methods with `*_for_target`.

We no longer look up a string in the config, but a section under "target". The
values in that section are simply passed to the Engine constructor.

Stuff still to do for this bit:

* Rename `db_uri` to `db`.
* Rename `destination` to `target`.
* Rename `core.$engine.db` to `core.$engine.target`.
* Rename `--database` to `--target` in status.
* Document all keys allowed in each target config.