If a payer is buggy and tries to pay the same invoice multiple times,
it can lead to an edge case where the recipient accepted the first one
and purchased liquidity for it, but didn't purchase additional liquidity
and thus cannot receive the duplicate payments.

In that case, the recipient may even have released the preimage, so the
LSP (or any other node in the route) could steal the entire duplicate
payment.

To prevent that, we immediately fail such buggy duplicate payments. Note
that if the first payment was fully completed, we don't fail the extra
payments and the payer could get screwed, but they're buggy so it's their
problem...

When replaying parts that were waiting for an on-the-fly funding, we
explicitly set `commit = false` to all parts but the last one, in order
to save us from signing multiple times.

But this has side effects in the case where the last part is rejected
for some reason, because previous parts stay stuck waiting for a
`CMD_SIGN` that never arrives.