routing: use Identifier in place of PaymentHash

Since we want to support AMP payment using a different unique payment
identifier (AMP payments don't go to one specific hash), we change the
nomenclature to be Identifier instead of PaymentHash.

routing/payment_session.go

@@ -171,7 +171,7 @@ func newPaymentSession(p *LightningPayment,
 		return nil, err
 	}
 
-	logPrefix := fmt.Sprintf("PaymentSession(%x):", p.PaymentHash)
+	logPrefix := fmt.Sprintf("PaymentSession(%x):", p.Identifier())
 
 	return &paymentSession{
 		additionalEdges:   edges,