- rename trampoline_forwardings -> final_onion_forwardings,
because this dict is used for both trampoline and hold invoices
- remove timeout from hold_invoice_callbacks (redundant with invoice)
- add test_failure boolean parameter to TestPeer._test_simple_payment,
in order to test correct propagation of OnionRoutingFailures.
- maybe_fulfill_htlc: raise an OnionRoutingFailure if we do not have
the preimage for a payment that does not have a hold invoice callback.
Without this, the above unit tests stall when we use test_failure=True
bf86cd6761