DPL: fix routing issues in forwarding

[ktf]

If one (header, payload, ...) tuple in a MessageSet was to be copied, all the subsequent ones would have been copied.

If one (header, payload, ...) tuple got redirected to more than one destination, all the subsequent ones would have been redirected there.

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• #14910 (2 commits)
• #14911 (2 commits)
• -> #14859 (2 commits)