Refactor Namespace and DynamicClassLoader statics
into new LoaderContext final class.

DynamicClassLoader:
- Add reference to context, inherited from parent.
- Remove unused constructor.
- Remove unused (!) constantVals.
- classCache per context.
- close() empties the context maps.
- Register as parallel capable.

Namespace:
- Remove use of WeakHashMap.

clojure.core:
- Remove *ns-root*, replaced by using 
  Thread context ClassLoader