gossiper: do_stop_gossiping: copy live endpoints vector
gossiper: do_stop_gossiping: copy live endpoints vector

It can be resized asynchronously by mark_dead.

Fixes #5701

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20200203091344.229518-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla