Add a configurable session idle timeout

[brainslog]

Currently it is possible that some sessions will live forever, due to external factors or even the nature of some applications. On the long run this could cause severe memory leaks and lead to the stack stop working properly.

It should be possible to define a Session Idle Timeout, where if the session had no activity during the specified timeframe, it will be terminated.

Optionally, it should be possible to configure this timeout on a Diameter app basis, so Ro might have different requirements than Sh, for instance.

Also as an option, listeners might get a callback for this and be able to act on it (eg, try to terminate the session gracefully).