Enhancements to PUBLISH management (thanks Johan Lantz for the suggestion)

[bennylp]

Several new enhancements for the PUBLISH handling (for presence):

• provide callback in PJSUA-LIB when PUBLISH session terminates, so that application may renew the session if necessary.
• handle some failure scenarios automatically, such as:
  • 412/Conditional Request Failed
  • 423/Interval Too Brief
  • 408/Request time out
• subsequent PUBLISH refresh should contain the custom headers that the application adds.

[bennylp]

In r2661:

• Decided to make the bulk of the changes in PJSUA-LIB instead of in publish session itself, to make the handling simpler.
• Decided to not provide callback in PJSUA-LIB, since we don't have API to force refresh anyway. Instead, PJSUA-LIB will retry failed request automatically (see below)
• Changes in PJSUA-LIB
  • retry with fresh request on 412/Conditional Request Failed response
  • changed default Expires in PUBLISH request to none (we will not put Expires), to avoid getting 423/Interval Too Brief response
  • if the PUBLISH fails for any reason, it will be retried on every PJSUA_PRES_TIMER (default 300 seconds), similar to how failed SUBSCRIBE will be retried
• Changes to publish.h:
  • added API to add headers in every PUBLISH request (pjsip_publishc_set_headers())
• Added test scenario in Python unit tests