Add caching to populateFacetValueCountMap in FacetedNavigationEngineThirdImpl

for example something like:

private WeakHashMap<IndexReader, Map<String, int[]>> cache = new WeakHashMap<IndexReader, Map<String, int[]>>();

Map<String, int[]> termDocCache = cache.get(indexReader);
if(termDocCache == null)

{ termDocCache = new HashMap<String, int[]>(); cache.put(indexReader, termDocCache); }

etc etc

note that it will be accessed concurrent