apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/grayReleaseRule/GrayReleaseRulesHolder.java/GrayReleaseRulesHolder/GrayReleaseRulesHolder
class GrayReleaseRulesHolder:  public GrayReleaseRulesHolder() {
    loadVersion = new AtomicLong();
    grayReleaseRuleCache = Multimaps.synchronizedSetMultimap(
        TreeMultimap.create(String.CASE_INSENSITIVE_ORDER, Ordering.natural()));
    reversedGrayReleaseRuleCache = Multimaps.synchronizedSetMultimap(
        TreeMultimap.create(String.CASE_INSENSITIVE_ORDER, Ordering.natural()));
    executorService = Executors.newScheduledThreadPool(1, ApolloThreadFactory
        .create("GrayReleaseRulesHolder", true));
  }