New congestion control metrics

@dgoulet and I are puzzling over why the average congestion control window is hovering around 400 on our test relays: tpo/network-health/analysis#44 (comment 2857941)

We're going to add at least 3-4 more metrics to help diagnose this:

• queue use
• Vegas BDP
• number of circuits that exit slow start
• Percent of circuits that exit slow start