unit tests: dnf.cli.format
drop per-repo interrupt callback

This is ignored in repo.set_interrupt_callback() and with multi-downloader
there's no "active" repo anyway. Callbacks (and progress meters, too)
must be global.
add dnf.cli.progress

MultiFileProgressMeter: this meter is used to display progress
of multiple downloads running in parallel.

LibrepoCallbackAdaptor: drop-in replacement of the old callback
adaptor for single-file downloads. The win here is we don't have
to import urlgrabber.progress anymore.
unit tests: dnf.cli.progress
use librepo.download_packages()

Moved most of the functionality to dnf.repo, download_packages()
is just few lines.  Dropped verifyPkg() call as librepo handles
checksumming of existing files already.