RELAY2: provide FIFO ordering for relayed messages

When relaying messages between (1) a local member and local coordinator (SiteMaster) and (2) between the SiteMaster of one site and the SiteMaster of a remote site, messages are by default FIFO-ordered as we're sending them on a JGroups channel.
However, when the target (coordinator or remote SiteMaster) changes, we need to replay unacked messages. If new messages are sent in parallel to the resending of unacked messages, ordering is undefined.

Investigate whether (a) ordering is needed (main consumer is Infinispan, so this is certainly not necessary for async repl, but may be needed for sync repl) and (b) how this should get implemented. Re (b): perhaps we can copy the queueing design from SEQUENCER (refactor the code into a separate class, used by both SEQUENCER and FORWARD_TO_COORD).