Analyze risks and benefits of clients having three guards

(Re-using Roger's text here.)

Based on the WPES 2012 paper "Changing of the Guards: A Framework for Understanding and Improving Entry Guard Selection in Tor", write a Tor proposal to pick safer parameters for guard selection (pick fewer guards, rotate them less often, etc). Then help with the follow-up paper to start exploring whether there are better algorithms to assign the Guard flag (to improve safety and performance).