Revert to UNICAST where RSVP is not supported

This is related to ISPN-1879. UNICAST2 without RSVP behaves quite badly when there are multiple senders and a single joiner, see ISPN-1838.

UNICAST2 + RSVP doesn't seem to perform much better than UNICAST, so rather than backporting RSVP to the JGroups 3.0.x branch it's probably safer to revert to UNICAST in the default Infinispan configuration.