Positions carry valuation and risk, computed once and served everywhere

Positions gain valuation and risk fields, so a client no longer has to price anything itself.

• usd_value, health_factor, borrow limit and liquidation price are returned per position.
• Valuation is attached to a position when it is recorded, so repeating a request returns the same numbers.
• Each valuation carries its vintage: which price it used and how old that price was.