Prefer outbound SCID alias over real SCID when routing

With splices, channels may spontaneously change SCIDs, making our assumption that SCIDs were more stable than SCID aliases in `ChanelDetails::get_outbound_payment_scid` wrong. We should swap which we prefer in `ChanelDetails::get_outbound_payment_scid`, but sadly doing so breaks a few tests that need fixing.