Before this change HandlerMethodReturnValueHandler's were invoked in a
specific order (type-based, annotation-based, custom). However handlers
that deal with asynchronous return value handling need to always be
considered first. This affects custom handlers in particular since they
are normally ordered last.
This change introduces an AsyncHandlerMethodReturnValueHandler
sub-interface with a single method to determine if the return value is
asynchronous and if it is to look for a matching handler only among
those that are of type AsyncHandlerMethodReturnValueHandler.
Issue: SPR-13083