Multiple discovery protocols - add MULTI_PING automatically

Previously, only one discovery protocol could have been used per stack. To allow for multiple discovery protocols in JGroups, MULTI_PING (JGRP-2224) protocol was implemented which did not require all protocols to be modified to support the change (especially since many protocols now reside outside the main jgroups project).

Though, this is a temporary measure which is planned to be remedied in JGroups 5.x with JGRP-2230 where MULTI_PING will no longer be required.

To remedy the inconvenience, we could add MULTI_PING automatically for JGroups 4.x release (in somewhat similar fashion FORK is added).