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.

Also check the correctness of timer heap operations and fix potential issues throughout the libraries.