Add xon+xoff counts vs stream counts to extra-info

In normal conditions, we do not expect flow control to activate. It should only happen if edge connections are somehow slower than Tor, or stalled. We will verify this in Shadow -- since Shadow does not rate limit tgen clients and webservers, but only rate limits relays, it should never see XON/XOFF, unless there are fairness issues between edges and orconns in Tor's mainloop, or TCP fairness issues between edges and orconns.

In the wild, things may be different. Shadow might not be fully simulating various aspects of TCP fairness wrt servers, or may not have sufficient upload traffic to trigger this property at Exits, as compared to the live network.

For this reason, it is probably wise to add a line to extra-info that includes the number of XOFF/XON sent, as well as the total number of streams, and the percentage (or counts) of streams that sent an XOFF or XONN vs those that did not. This way, we can confirm that what we see in Shadow is the same as what happens in live, which will also be useful for future work.

Cc: @dgoulet