Include invreq in payment onion when sending async payments

Include invreq in payment onion when sending async payments

Past commits have set us up to include invoice requests in outbound async
payment onions. Here we actually pull the invoice request from where it's
stored in outbound_payments and pass it into the correct utility for inclusion
in the onion on initial send.

Per BOLTs PR 1149, when paying a static invoice we need to include our original
invoice request in the HTLC onion since the recipient wouldn't have received it
previously.