new: scanner: do HTTP POST requests instead of GET

when the consensus parameter bwscanner is 2.

"Using upload instead of download allows the congestion control algorithms to be under control of the sbws Tor client, instead of the Exit relay. This means that sbws can know when slow start has exited."

This is the 1st optimization for #40130 (closed).

The testing HTTP server has been replaced by the one implemented in onbasca just cause a first try on implementing do_POST was successful only for the 1st sbws request. It only implemnts POST RFC 2388.

Co-authored-by: Georg Koppen gk@torproject.org