FieldDictionary.buildMap does not scale well in concurrent environments

Having many threads concurrently using XStream, we run into scalability issues because of the FieldDictionary. The current implementation still blocks for a cache hit.

I attached a patch to this issue, using double checked locking and a ConcurrentHashMap to overcome this issue. Using ConcurrentHashMap should be an option for Java 1.5, which is hopefully the target for the current trunk.

With this patch I also optimized the traversal upwards the type hierarchy. In my opinion traversal could be stopped if a cache entry for a superclass is found.