listen: disarm the timers when closing a virtual listener

listen: disarm the timers when closing a virtual listener

proto_load_step and proto_cron_crontab drive themselves from timers rather than from their fd, and neither had a close callback, so nothing stopped them when the listener was closed. The load generator kept manufacturing packets and handing them to fr_network_send_request() after the network had signalled the workers to close, which the assert added there caught.

Both now close the fd and remove their timer.  Of the remaining app_io modules with timers, proto_detail_file, proto_detail_work and proto_ldap_sync_ldap have a close callback that does not obviously disarm anything, and are worth a look.