private int countUnique(Map<K, V> large, Map<K,V> small) {
        int size = large.size();
        for (K key : small.keySet()) {
            if ( ! large.containsKey(key)) size++;
        }
        return size;
    }
A read-only map which forwards lookups to a primary map, and then a secondary for
keys not existing in the primary.

@author bratseth
container-search/src/main/java/com/yahoo/search/query/profile/ChainedMap.java