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.
Stupid idea, but we're relying on times, should we be relying on message id? Ask datagrepper for all the messages that occurred after a specified ID?