boolean incNextCallSeq(long currentSeq) {
      // Use CAS to prevent multiple scan request running on the same scanner.
      return nextCallSeq.compareAndSet(currentSeq, currentSeq + 1);
    }
Holder class which holds the RegionScanner, nextCallSeq and RpcCallbacks together.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java