public CompletableFuture<Boolean> start(HBaseRpcController controller,
    ScanResponse respWhenOpen) {
    onComplete(controller, respWhenOpen);
    return future;
  }
Now we will also fetch some cells along with the scanner id when opening a scanner, so we also
need to process the ScanResponse for the open scanner request. The HBaseRpcController for the
open scanner request is also needed because we may have some data in the CellScanner which is
contained in the controller.
@return {@code true} if we should continue, otherwise {@code false}.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncScanSingleRegionRpcRetryingCaller.java