As part of vAMM, remove the part of the margin calculation where we estimate slippage from order book...

[davidsiska-vega]

... and allow lower margin based on that.

I.e. the margin will be based on (linear slippage factor + rf) x price x size

And max leverage is simply 1/ (linear slippage factor + rf)

[gordsport]

@peterbarrow / @wwestgarth - please can we do some performance review/thinkings and see if this is likely to be required