Debugging aid for timers and fix timer issues

[bennylp]

Add facility to debug timer heap state. Application can declare PJ_TIMER_DEBUG=1 and recompile, and use pj_timer_heap_dump() to dump the contents of the timer heap. The contents of the timer heap will also be printed to log by pjsip_endpt_dump(), and when the pjsip_endpt is being destroyed.

[bennylp]

In 4154:

Re #1527: added debugging facility to the timer heap. By enabling PJ_TIMER_DEBUG, application can use pj_timer_heap_dump() or pjsip_endpt_dump() to dump the timer entries along with the source location where it is scheduled from. The macro will also enable dumping the timer heap entries when the SIP endpoint is being destroyed

[bennylp]

In 4155:

Re #1527: fixed bug in pjsua-lib, where registration operation may cause wrong timer entry (with _timer_id=0) to be cancelled accidentally. This could happen when turning registration ON after it was off. Thanks Itay Bianco for the report