pjproject_bundled: timer: Clean up usage of timer heap

Added a new pj_timer_entry_reset function that resets a timer_entry
for re-use.

Changed direct settings of timer_entry fields to use
pj_timer_entry_init and pj_timer_entry_reset.

Fixed issues where timers were being rescheduled incorrectly.

Change-Id: I5b624bfbc5c1429117484b9b24567293002148e6