Preserve message metadata during a node-to-node transfer

[MHillyer]

As a MailOps professional, I want to be able to scale my nodes automatically by transferring messages between nodes.

When performing the transfer, I don't want the message metadata to be lost. I need to know that the message's expiry doesn't reset with its arrival at a new node, and that it's next retry doesn't change, resulting in a delivery attempt burst when the messages move to other nodes.

In addition, if I've read headers on initial injection, moved their data to metadata, and then removed those headers, that metadata can't be lost during a node-to-node transfer.