void freezeMutableConf() {
    if (mutableConf == null) {
      // do nothing if there is no current mutableConf
      return;
    }

    this.configs.add(0, new ImmutableConfWrapper(mutableConf));
    mutableConf = null;
  }
If set has been called, it will create a mutableConf. This converts the mutableConf to an
immutable one and resets it to allow a new mutable conf. This is used when a new map or conf is
added to the compound configuration to preserve proper override semantics.
hbase-common/src/main/java/org/apache/hadoop/hbase/CompoundConfiguration.java