MINOR: threads: Define the sync-point inside run_poll_loop

MINOR: threads: Define the sync-point inside run_poll_loop

The function sync_poll_loop is called at the end of each loop inside
run_poll_loop function. It is a protected area where all threads have a chance
to execute tricky tasks with the warranty that no concurrent access is
possible. Of course, it comes with a cost because all threads must be
syncrhonized. So changes must be uncommon.