routing/payment_lifecycle: move requesting route out of createNewPaymentAttempt
routing/payment_lifecycle: move requesting route out of createNewPaymentAttempt

To prepare for having more than one payment attempt in flight at the
same time, we decouple fetching the next route from crafting the payment
attempt.

aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd