Use JGroups' anycast when sending messages to multiple target

We currently emulate anycasts in CommandAwareRpcDispatcher by sending the message to each target sequentially and then waiting for all the responses in parallel.

This doesn't work very well with JGroups' RSVP protocol: if the RSVP flag is set, the send operation will block until we have received an ACK from the target, making the operation take much longer than it should.

We only use RSVP for state transfer-related commands, so this is not critical. But it should be slightly more efficient for normal commands, as well.