Handle backlog on startup.

It makes it so that we can restart fedmsg-processes and have them pick up
where they left off... which should be pretty useful for more quickly
debugging and restarting the fmn backend.

This depends on mokshaproject/moksha#17.

This fixes fedora-infra/fedbadges#6.
Unfortunately, that's not part of datagrepper's API.

Furthermore, it would give a false sense of certainty.  The messages are not guaranteed to arrive at datanommer in the same order that they arrived here.