Right now, the NodeRelayer indexes child actors by payment_hash only.
It means that senders must know beforehand the total amount they will send through each trampoline node.
This works fine for single-trampoline scenarios, but will not work with multi-trampoline scenarios: if the payer splits his payment between several trampoline routes and one fails, he may end up sending more additional HTLCs to a previous route when retrying.
Right now, we will reject them because the payment_secret doesn't match, or we'll simply fail to relay them (if the payment_secret is reused) and wait for a timeout from the final recipient.

We should instead index by payment_hash || payment_secret, but that requires extracting the payment_secret a bit higher up in our call stack.