code: String getNamespacedKey(CacheNamespace cacheNamespace, String key) {
          String namespaceIndicator = getNamespaceIndicator(cacheNamespace);
  
          if (cacheNamespace.isNamespaceExpirationRequired() == false) {
              return namespaceIndicator + ":" + key;
          }
  
          String namespaceIndicatorKey = namespaceIndicator + NAMESPACE_NAME_SQUENCE_SEPARATOR;
          long namespaceSquence = memcachedClient.incr(namespaceIndicatorKey, 0L, System.currentTimeMillis(),
                  DEFAULT_NAMESPACE_SEQUENCE_EXPIRY_SECONDS);
  
          return namespaceIndicatorKey + namespaceSquence + ":" + key;
      }
docs: Return cache namespace decorated key.
  
  @param cacheNamespace cache namespace
  @param key cache key
  @return namespace infomation prefixed cache key