So it would be waited on in shutdown().
Although gossiper::run holds the `_callback_running` semaphore
which is acquired in `do_stop_gossiping`, the gossip messages
it initiates in the background are never waited on.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closesscylladb/scylladb#15493