Set individual message delays

[stanley-travelex]

I've just started using the new feature to add delays in #76 but I'd like to set the delay on a per message basis instead of the whole MessageProducer.

I've created #79 to add support for this, where we can set a message property to override the delay for the MessageProducer.

Let me know what you think, cheers!
Stanley

[Flying-Bison]

Is there any consideration to include this option?

While I think the change in #76 was important, I think this feature is also needed. These are two different scenarios and some use cases are not covered without this change.

Thank you for any information you can provide. I'd really rather use JMS instead of the SQS objects in the AWS SDK directly.