HSEARCH-1296 JGroups backends always behaves like an ASYNC backend


@alesj : careful as the code change won't be effective without configuration changes.
It turns out JGroups needs to have the RSVP protocol with ack_on_delivery=true .
According to @danberindei you should not enable this option on the JGroups stack used for Infinispan: make sure you have two separate channels or have him explain alternatives?