Clustered event-bus shutdown frame

When a clustered event-bus is shutdown, it will close all its connection to its peers. We could introduce a shutdown sequence, that sends a close frame before TCP connection close that indicates the server will not be available anymore.

This would allow peers to fail eagerly the pending requests and perhaps other use cases.

This would require a modification of the event-bus protocol so it would be a breaking change.

See https://github.com/eclipse-vertx/vert.x/issues/4183