Avoid storing a full FinalOnionHopData in OnionPayload::Invoice

Avoid storing a full FinalOnionHopData in OnionPayload::Invoice

We only use it to check the amount when processing MPP parts, but
store the full object (including new payment metadata) in it.
Because we now store the amount in the parent structure, there is
no need for it at all in the `OnionPayload`. Sadly, for
serialization compatibility, we need it to continue to exist, at
least temporarily, but we can avoid populating the new fields in
that case.