void clearWeightCache() {
    lastWeighedAt = -1;
  }
Generates a balancing action to appease the conditional.
@param cluster    Current state of the cluster.
@param isWeighing Flag indicating if the generator is being used for weighing.
@return A BalanceAction, or NULL_ACTION if no action is needed.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/RegionPlanConditionalCandidateGenerator.java