Adjust index refresh period as INT(flushInterval * 2/3), it used to be as same as bulk flush period. At the edge case, in low traffic(traffic < bulkActions in the whole period), there is a possible case, 2 period bulks are included in one index refresh rebuild operation, which could cause version conflicts. And this case can't be fixed through core/persistentPeriod as the bulk fresh is not controlled by the persistent timer anymore.
This PR should avoid the following exception in the low load case, especially when bulkActions is set larger than the number of a metric type.
8df362b929