Use the exits that implement congestion control as second hop

This issue is the equivalent of sbws#40125 (closed):

• If cc_alg=2 is present in the consensus, sbws should only use Exits with FlowCtrl=1-2 as second hop

That will get us improved measurement, but may have some sub-optimal performance issues. Three optimizations are:

• sbws should use upload to bwauth http servers, instead of download
• sbws should track the CIRC_BW event to only start measuring upload bandwidth once the CIRC_BW field SS=0.
• sbws should monitor STREAM events during upload, and if it ever gets STREAM STATUS=XOFF_RECV, it knows that the bwauth httpd server is too slow to properly measure some relays