now, don't know why, several witness publish feed price double
for CNY, the normal price is 0.025, but some witness publish 0.049.
this can be a very serious security problem.
if more witness give wrong price, we will get a final feed price at 0.049,
users can borrow CNY at price 0.049, the collatereal is not enough, a black swan will happe.
I don't want to talk about why they give a wrong feed price, maybe it's a bug of feed script,
maybe an exchange return a wrong price
the more important thing is we should avoid this happen even if the witness give a wrong feed price.
so I ask for a new limit at the borrow logic
currently when we borrow from the wallet, the collatereal's price is the feed price.
we should get it from min(feed price, highest bid price)