multi: validate payment params at RPC layer

With this patch, we'll fail out earlier in the cycle in case of
some wonky parameters, and not leave zombie payments in the router
which currently are not cleaned up.

routing/payment_session.go

@@ -14,6 +14,22 @@ import (
 // to prevent an HTLC being failed if some blocks are mined while it's in-flight.
 const BlockPadding uint16 = 3
 
+// ValidateCLTVLimit is a helper function that validates that the cltv limit is
+// greater than the final cltv delta parameter, optionally including the
+// BlockPadding in this calculation.
+func ValidateCLTVLimit(limit uint32, delta uint16, includePad bool) error {
+	if includePad {
+		delta += BlockPadding
+	}
+
+	if limit <= uint32(delta) {
+		return fmt.Errorf("cltv limit %v should be greater than %v",
+			limit, delta)
+	}
+
+	return nil
+}
+
 // noRouteError encodes a non-critical error encountered during path finding.
 type noRouteError uint8