code: public synchronized void updateMasterConf(PropertyKey key, @Nullable String value) {
      mMasters.forEach(master -> master.updateConf(key, value));
    }
docs: Updates master configuration for all masters. This will take effect on a master the next time
  the master is started.
  
  @param key the key to update
  @param value the value to set, or null to unset the key