public void freeze() {
        if (frozen)
            throw new IllegalStateException("The map has already been frozen");
        frozen = true;
        map = Collections.unmodifiableMap(map);
    }
Delegates to a map that can be frozen.
Not thread safe.

@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/utils/FreezableMap.java