public void attachCompletionAction(Runnable action) {
      assert !completionAction.isPresent();
      completionAction = Optional.of(action);
    }
Will be called after completion, i.e, when being removed from the
{@link MultiVersionConcurrencyControl#writeQueue}.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MultiVersionConcurrencyControl.java