Document the replay mechanism and how to stop it

[pypingou]

Fedmsg-hub stores on disk in /var/run/fedmsg/status/fedmsg-hub/* the last fedmsg message received by every consumer it is running. This allows the consumer to go and query datagrepper for any newer messages upon restart.

To stop this behavior (just for one restart), simply nuke this file, fedmsg-hub will not know how long it has to go back and ask datagrepper and thus will not do it.

[ralphbean]

Note that you have to 1) stop the hub, 2) rm the file, 3) restart the hub.

If you just nuke it and restart, it might write out another status file right after you nuked it.

[ralphbean]

The code is here.

[cha-ku]

I'll take this up.

[jeremycline]

@Chaiku great!

Just send in a PR - I can't assign the issue to users outside the Github organization.