RELAY2: messages lost when local site master crashes

When we have sites LON=

and SFO=

, if C wants to send a unicast message to the site master of SFO (X), but the local site master (A) leaves or crashes, and B hasn't taken over yet, the message will be lost.

The idea to solve this is to forward the message to the next coordinator if the current coordinator leaves or dies.

A FORWARD_TO_COORD protocol was developed, which handles this task. RELAY2 checks at startup if FORWARD_TO_COORD is present and uses a FORWARD event to tell that protocol to forward a message to the current coordinator. If the protocol is not present, a simple unicast will be sent (unreliably).

FORWARD_TO_COORD sends a message M to the current coord and removes M when an ack has been received. If there is a view change, indicating the old coord left, it resends all pending messages, and so on. The extreme case would be that everyone but the sender dies and then M would be sent to the sender itself.