[Rpm-ecosystem] Patch review request: zchunk patches for dnf, libsolv and librepo

[Jonathan Dieter]

I've finally finished writing patches to integrate zchunk support into
dnf/libsolv/librepo[1], and I'd greatly appreciate some code review.  A
vast majority of the code is in librepo, but libsolv has been expanded
to support zchunk files and dnf has a tiny patch that passes the base
cache directory to librepo to find source zchunk files to delta
against.

With these patches and a zchunk-enabled repository, you will download
only the differences in your primary/filelists/other metadata.  Partial
downloads are validated and then continued, and each chunk is validated
as it's downloaded, ending the download and moving to a new mirror if
any chunk is corrupt.

I would love to get these changes into Fedora 29, and the code is
testable now, but with only three weeks until System-Wide change
proposals are due, I'm not sure if I'm being ambitious.

On another note, I would also like to finalize zchunk's API and make a
stable ABI promise, but before I take that step I'd really love some
feedback on its usability.

Jonathan

[1] https://github.com/rpm-software-management/dnf/pull/1107
    https://github.com/openSUSE/libsolv/pull/270
    https://github.com/rpm-software-management/librepo/pull/127