Test a 4.2 release with a multiplexer sar

Plug in JGroups 2.5 or later, add a multiplexer sar and run the testsuite. Report any issues and add a wiki page explaining how to do it.

This would be far from a full QA of the use of the multiplexer. No such sar would be include in any official 4.2 distro (see comments on parent issue for why not.) There would be no intent to try to support such usage (and the wiki must say that.) This is more in the spirit of providing a bit of info to community users who may want to try it themselves, at their own risk.