Improve the risk score calculation

[toneloc]

Right now the "risk_score" for a counterparty is incremented by 1 each time a user's expected dollar amount is not within $0.01 ... be that user the Stable Receiver or Stable Provider.

It would be better is this was more sophisticated. It could be amount-based or take into account repeated offenses.

We could change risk_score in several ways, for example:

• so that a difference of 1% is 100x riskier than a difference of 0.01%
• so that after several incorrect settlement events in a row ... e.g. 15+ minutes of being off by +1% ... the risk_score estimate accelerates
• so that it accounted for incorrect settlements across days or weeks
• so that it would be difficult for a misbehaving user to fool the risk_score