feature: use market analysis instead of xdaythreshold

[tisdall]

This is partially related to #244 but I think this is better functionality.

Right now we have market analysis determining when the rate is too low to loan out coins. You can configure it with a percentile. Instead of using a global static loan percentage, it'd be nice to say (for example) when the current loan rates are in the 95th percentile to loan for xdays. This would allow you to lock in loans for longer terms when the rates are at an all-time high.

[laxdog]

I like this idea.