gossip: Do not stop gossip more than once
gossip: Do not stop gossip more than once

If we do
   - Decommission a node
   - Stop a node
we will shutdown gossip more than once in:
   - storage_service::decommission
   - storage_service::drain_on_shutdown

Fix by checking if it is already stopped and back off if so.

scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla