Use Message.Flag.DONT_LOOPBACK instead of RequestOptions.setExclusionList

We are currently using RequestOptions.setExclusionList(channel.getAddress()) in CommandAwareRpcDispatcher, to prevent the sender of an RPC to receive that message. We can't use Channel.setDiscardOwnMessages(true), because TO requires messages to be delivered to self.

JGroups 3.5.0.Beta2 has added a DONT_LOOPBACK flag, which should be a lot more efficient than using the exclusion list. We should start using it.