Support for SIP Session Timer (RFC 4028)

[bennylp]

Scope:

• Run-time configurable session interval (Session-Expires header) and minimum session interval (Min-SE header) per account basis.
• Refresher performs refreshes at half of session interval, and refreshee sends BYE slightly before session expiration. 'Slightly' == minimum of 32 seconds and one-third of session interval.
• Session interval negotiation, i.e: generate/handle 422 when SE less than Min-SE.
• Works even when only one UA in a dialog supports Session Timer (and it's not listed in Require header).
• Able to use Session Timer even when peer doesn't support Session Timer, perform refreshes by and for itself.
• Able to adapt when Session Timer is turned-off.

Limitations:

• Refreshing session using UPDATE hasn't been supported yet.
• Refreshing session will always restart the media session (even if the media session is not modified).
• Session Timer can always be listed in Supported header, but it's not necessarily always used (by omitting Session-Expires header), currently there is no mechanism to omit/turn-off Session-Expires header.

[nanang]

In r2858:

• Initial version of Session Timers (RFC 4028).
• Added new options in pjsua app to configure Session Timers settings.
• Added python tests for Session Timers.