Allow setting a delay header on a message

[nirik]

There is a delayed message plugin for rabbitmq:
https://github.com/rabbitmq/rabbitmq-delayed-message-exchange

It would be cool if you could set these headers for messages you might want to delay.

This came up in a discussion about the koji messaging plugin sending a 'build done' message but then it going through so fast it's not appeared on the nfs volume on another hub yet. ( https://pagure.io/koji/issue/3178 )

But there could be a bunch more cases for this too...

[abompard]

If I'm reading the plugin docs well, we would need to setup an additional exchange for delayed messages, and bind the queues to this exchange. This would be pretty cumbersome, and in addition the delayed messages are not replicated through the cluster.

I'm not sure this is the best solution for this issue. I would recommend that the consumer polls for the file on the NFS volume at regular intervals when they receive the message, this way the sender will not have to guess how long the file will take to appear on every consumer.