Adaptable insane fee sanity check

[darosior]

The point of the insane fee sanity check is to be a belt-and-suspender in case our feerate computation is wrong.

However, 0.2BTC of fees may be reasonable for, say, a hundred-inputs Spend transaction (w/ many outputs too). Maybe we could have a check depending on the amount spent? As a sweet pot between straightforwardness of the check and adaptability to many transaction.