Simplify RspFilter interface

RspFilter interface although having only two methods in its contract has been a source of some confusion regarding the semantics of response filtering. In order to simplify and make response filtering semantics easier to understand and implement a new RspFilter has been proposed for 3.0.

RspFilterResult responseReceived(Object response, Address sender);

RspFilterResult is an enum with four states where each state is essentially a combination of two boolean variables: validResponse and needMoreResponses.

Original discussion reference:
http://sourceforge.net/mailarchive/forum.php?thread_name=4BC8943F.2000101%40yahoo.com&forum_name=javagroups-development