GroupRequest: don't send target member list with each request

Currently, if we invoke an RPC (RpcDispatcher) or a sync group message (MessageDispatcher), we always send the target membership list with the request.

The reason is that a member receiving a group request can drop it if it is not in the target list. However, for group requests, usually all of the members are in the target list anyway, and responses sent from members not in the list would get dropped anyway, as the GroupRequest itself keeps track of members from which responses are supposed to be received.

For large clusters, the overhead of shipping the target list is too big, e.g. for 100 nodes, the cost of 100 addresses (UUIDs) is roughly 2000 bytes !

SOLUTION: drop the target membership list