@Override
    public V get(Object key) {
        V value = primary.get(key);
        return value != null ? value : secondary.get(key);
    }
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