Add constant and percentage-based fee limits to payments

- Extend SendRequest and QueryRoutesRequest protos
- newRoute function takes fee limit and cuts off routes that exceed it
- queryRoutes, payInvoice and sendPayment commands take the feeLimit inputs and pass them down to newRoute
- When no feeLimit is included, don't enforce any feeLimits at all (by setting feeLimit to maxValue)

routing/errors.go

@@ -42,6 +42,10 @@ const (
 	// ErrPaymentAttemptTimeout is an error that indicates that a payment
 	// attempt timed out before we were able to successfully route an HTLC.
 	ErrPaymentAttemptTimeout
+
+	// ErrFeeCutoffExceeded is returned when the total fees of a route
+	// exceed the user-specified maximum payment for the fee.
+	ErrFeeCutoffExceeded
 )
 
 // routerError is a structure that represent the error inside the routing package,