output: show the RPMDB alternation warning on debug level only.

this is internal and should not scare the user.
implement dnf.repo.Repo.__repr__().
add noop NoOutputCallback.verify_tsi_package().

it's really no use without it.
Take out the bulk of YumBaseCli.doTransaction() and put in in Base.do_transaction().

The part that can reasonably by used by API clients should reside in Base.