Message4191

Given that PyStringMap proxies ConcurrentMap<Object, PyObject> (in order
to handle both interned String and PyObject instances), we should be
able to implement the ConcurrentMap interface just like PyDictionary.
Then we can apply Alan's fix.

Here are the missing methods from ConcurrentMap. We may also have some
methods necessary for dict usage, although I didn't see any (what tests
specifically test all of __dict__ ?)

    public Object putIfAbsent(Object key, Object value) 
    public boolean remove(Object key, Object value) {
    public boolean replace(Object key, Object oldValue, Object     value)
    public Object replace(Object key, Object value) 
    public int size()
    public boolean isEmpty()
    public boolean containsKey(Object key)
    public boolean containsValue(Object value)
    public Object get(Object key)
    public Object put(Object key, Object value)
    public Object remove(Object key)
    public void putAll(Map t)
    public Set keySet()
    public Set entrySet()