Execute all the due wakeup events in SipWakeupTimer.

Events are sorted by periods. So events of larger periods may have trigger
time (i.e., when the event should be processed) earlier than the ones of smaller
periods. So need to scan the whole queue looking for due events. The scan takes
O(n) time but we expect the queue size to be small.

Change-Id: I08bd3bd9d4bb8decb78f3c91c943396463ca023a