Extend the new Python Torperf to talk to tor's control port

After finishing #3281 (closed), we should extend Torperf to talk to the control ports of the tor processes it started before. Torperf should use TorCtl for this task similar to extra_stats.py and entrycons.py. In fact, we can probably re-use most of the code from these two files.

Connecting to tor's control port has two purposes that have to be performed by the new Torperf: First, Torperf may have to influence tor's guard node selection algorithm depending on its configuration, and second, Torperf will have to process control port events and write them to a local .extradata file.

Assigning to tomb upon request.