Move the `raft_group_registry::on_alive` error injection point
to `gossiper::real_mark_alive` so it can delay marking the endpoint as
alive, and calling the `on_alive` callback, but without holding
the endpoint_lock.
Note that the entry for this endpoint in `_pending_mark_alive_endpoints`
still blocks marking it as alive until real_mark_alive completes.
Fixes#16506
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
db434e8cb5