Race condition after shutdown gossip message

CASSANDRA-8336 introduced a race condition causing gossip messages to be sent to shutdown nodes even if they have been already marked dead.

That's because CASSANDRA-8336 changed (among other things) the way the SHUTDOWN gossip message is sent by moving it before the gossip task (the one sending SYN messages), and by putting a few secs wait between the two; this opens a race window by the receiving side between the time the SHUTDOWN message is received, causing the outbound sockets to be closed, and the moment the other side listening socket is actually closed, meaning that any SYN gossip message exchanged in such window will reopen the sockets and never close them again, as the node is already marked dead.