A joining node hung forever if the topology coordinator added it to the
group 0 configuration before the node reached `post_server_start`. In
that case, `server->get_configuration().contains(my_id)` returned true
and the node broke out of the join loop early, skipping
`post_server_start`. `_join_node_group0_started` was therefore never set,
so the node's `join_node_response` RPC handler blocked indefinitely.
Meanwhile the topology coordinator's `respond_to_joining_node` call
(which has no timeout) hung forever waiting for the reply that never came.
Fix by only taking the early-break path when not starting as a follower
(i.e. when the node is the discovery leader or is restarting). A joining
node must always reach `post_server_start`.
We also provide a regression test. It takes 6s in dev mode.
Fixes SCYLLADB-959
Closesscylladb/scylladb#29266
b9f82f6f23