Dom4j concurrency problem

[jbennett2091]

This is related to Issue #15

The concurrency problem that has been previously raised is caused by misuse of SynchronizedMap. As noted in SynchronizedMap's javadoc, a synchronized-block must be placed around things that iterate the map (c.f. https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#synchronizedMap-java.util.Map-). The QNameCache.getQNames() method fails to do this resulting in the possibility of ConcurrentModificationExceptions if getQNames() is iterating in thread#1 while thread#2 is mutating the map.

The use of a ConcurrentHashMap (as proposed as an enhancement in #15) would address this, but is complicated by a desire to preserve the WeakReferences within the map. That is, a ConcurrentWeakHashMap would solve it, but that is a heavy lift.

Instead, I would propose we simply follow the recommendation of SynchronizedMap. Namely:

    public List<QName> getQNames() {
        List<QName> answer = new ArrayList<QName>();
        synchronized(noNamespaceCache) {
            answer.addAll(noNamespaceCache.values());
        }

        synchronized(namespaceCache) {
            for (Map<String, QName> map : namespaceCache.values()) {
                answer.addAll(map.values());
            }
        }

        return answer;
    }

This will eliminate the bug and make Dom4j thread-safe. Implementing #15 in the future would additionally make it performant.