Major Description
The RestContext class should be subclassible so that behavior can be modified at runtime.
It could be selected on the builder class like so:
restContextBuilder.contextClass(MyRestContext.class)
The context class could also be defined via an annotation:
@Rest(context=MyRestContext.class)
This could also make classes like the RestCallHandler interface obsolete since these could all be defined as methods on the RestContext class and overridable there.