public void ack(SnapshotPacket packet) {
      FlowController.getInstance().sendAck(region.getDistributionManager(), packet.getSender(),
          packet.getWindowId(), packet.getPacketId());
    }
Aborts any further collection of results and forwards the cancellation to the members
involved in the export.
geode-core/src/main/java/org/apache/geode/internal/cache/snapshot/WindowedExporter.java