private ExtendedCellScanner getAndReset(RpcController controller) {
    HBaseRpcController hrc = (HBaseRpcController) controller;
    ExtendedCellScanner cells = hrc.cellScanner();
    hrc.setCellScanner(null);
    return cells;
  }
Warmup a region on this server. This method should only be called by Master. It synchronously
opens the region and closes the region bringing the most important pages in cache.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java