Deploy a bandwidth scanner that supports congestion control to check that it's used properly to the 2nd hop

As @mikeperry commented in IRC, we could deploy the bandwidth scanner (sbws or onbasca) in a testing machine to check that congestion control is negotiated to 2nd hop and not the 1st, since C-Tor negotiates it to the last Exit hop, but the bandwidth scanners are building the circuits themselves with the EXTENDCIRCUIT control port command.

For this, we would need to:

• force bwscanner >= 1 in the bandwidth scanner code
• @mikeperry would have a C-Tor branch with extra logging enabled