Release IP address on session stop

[Zensey]

Symptoms:

invalid config to consumer request with nil instead of ip address. like this: {"local_port":0,"remote_port":0,"ports":null,"provider":{"public_key":"iixI/brFnGk5JPADWVhzum2WltsMV2A8iabJKgJuWQo=","endpoint":"46.14.127.237:57757"},"consumer":{"ip_address":"\u003cnil\u003e","dns_ips":"\u003cnil\u003e"}}

Cause: IP is allocated for consumer on session start, but never gets released. Given the limited size of IP pool, exhaustion can happen in a few days or hours.