Fix exceptions at launch, post-crawl-finish

* KryoBinding.java
    always default to registrationOptional, for now
* StatisticsTracker.java
    explicitly set valueClass of sourceDistribution to ConcurrentHashMap
* StoredQueue.java
    answer 0 for size() when backing DB closed

commons/src/main/java/org/archive/bdb/KryoBinding.java

@@ -53,6 +53,8 @@ public class KryoBinding<K> implements EntryBinding<K> {
     public KryoBinding(Class baseClass) {
         this.baseClass = baseClass;
         kryo.autoregister(baseClass);
+        // TODO: reevaluate if explicit registration should be required
+        kryo.setRegistrationOptional(true);
     }
 
     public Kryo getKryo() {

commons/src/main/java/org/archive/bdb/StoredQueue.java

@@ -91,7 +91,11 @@ public class StoredQueue<E extends Serializable> extends AbstractQueue<E>  {
 
     @Override
     public int size() {
-        return queueMap.size();  
+        try {
+            return queueMap.size();  
+        } catch (IllegalStateException ise) {
+            return 0; 
+        }
     }
     
     @Override