Create log file for OnionPerf's own log messages

OnionPerf logs a couple of log message to the console and writes a few other messages to the torctl.log file. We should create a new log file for messages coming from OnionPerf itself. This log file should be placed in onionperf-data/, so that we can archive it together with the other log files. It should probably be rotated, just like all other log files are. And it should contain the OnionPerf version writing this log file.