Add repository "summary" file and metalink support

Add repository "summary" file and metalink support

For Fedora and potentially other distributions which use globally
distributed mirrors, metalink is a popular solution to redirect
clients to a dynamic set of mirrors.

In order to make metalink work though, it needs *one* file which can
be checksummed.  (Well, potentially we could explode all refs into the
metalink.xml, but that would be a lot more invasive, and a bit weird
as we'd end up checksumming the checksum file).

This commit adds a new command:

$ ostree summary -u

To regenerate the summary file.  Can only be run by one process at a
time.

After that's done, the metalink can be generated based on it, and the
client fetch code will parse and load it.

https://bugzilla.gnome.org/show_bug.cgi?id=729585