Delivery age tracking in the router

Implement a lightweight (one-second granularity) timer for deliveries to identify deliveries that remain unsettled or undelivered for an abnormally long time.

Add counters to links (and in the rolled-up general counter set) to count settled deliveries that were:

• unsettled for between 1 and 10 seconds
• unsettled for longer than 10 seconds

Add the above new counters to qdstat so they can be seen in the -l and -g options.