Add Support for Session-Timers

[cruzccl]

Once established, calls may last 3 hours (dialog default_timer parameter).

Make tests to implement session-timers in AS.

[Kaian]

In fact, this is enabled by default in asterisk 13.

According to the pjsip show endpoint output, the current enabled Session-timers for all peers are:

 timers                             : yes
 timers_min_se                      : 90
 timers_sess_expires                : 1800

This seems to work, but we require more testing to check if this behave like the sip analog with accept, originate, uac, uas, and so on.

[Kaian]

Two cents more:

According to Asterisk 13 Documentation these are the settings configurable by endpoint:

timers_min_se: Minimium session timer expiration period. Time in seconds. timers: [no, yes, required, always, forced] timers_sess_expires: Maximium session timer expiration period. Time in seconds.

[Kaian]

After some testing with pjsip settings I'm not able to archive the ideal session timers behaviour.

Ideal timers will be:

• Acting as UAS: suggest refresher if UAC supports timers. If it doesn't support them, still send them periodic reinvites to refresh the session (and check if it's still there)
• Acting as UAC: always suggest session-timers. If UAC rejects them, still send them periodic reinvites to refresh the session (and check if it's still there)

I was unable to make it work that way so let's delay this and give it a try in asterisk 14.