Before the patch if a decommissioned node tries
to restart, it calls _group0->discover_group0 first
in join_cluster, which hangs since decommissioned
nodes are banned and other nodes don't respond
to their discovering requests.
We fix the problem by checking was_decommissioned()
flag before calling discover_group0.
fixesscylladb/scylladb#17282Closesscylladb/scylladb#17358