relay: Add congestion control stats to MetricsPort

I will quote @mikeperry from IRC:

<+mikeperry> dgoulet: so a counter at the top of dos_note_circ_max_outq(), and a moving average of cc->cwnd in 
             congestion_control_vegas_exit_slow_start() are the two most useful easy additions to metricsport that
             I can think of for CC.. other points would be harder because they happen multiple times per circuit.
<+mikeperry> dgoulet: maybe a moving average of cc->cwnd upon circuit close?
<+mikeperry> dgoulet: oh hrmm.. I wonder if the Exit DoS is causing more XON/XOFF, actually
<+mikeperry> dgoulet: I suppose we could also count how often cc->vegas_params.delta and  cc->vegas_params.ss_cwnd_max
             get hit in congestion_control_vegas_process_sendme() if you are still on a metricsport rampage. just a
             raw counter there might be interesting, and easy