QUIC: fixed removal of unused client IDs. If client ID was never used, its refcount is zero. To keep things simple, the ngx_quic_unref_client_id() function is now aware of such IDs. If client ID was used, the ngx_quic_replace_retired_client_id() function is supposed to find all users and unref the ID, thus ngx_quic_unref_client_id() should not be called after it.