Minor improvement for fee estimation #10625

Volatility of fees required has increased significantly. Estimates already take a look at the mempool min fee and never return anything less than that. However the mempool min fee decays quite quickly, but I think it makes more sense to track a recent high for the mempool min fee and never return an estimate ~more~less than that. Probably only when conservative = true. Perhaps a history as long as 4 weeks?
Maybe we could just keep a rolling history of the max per day for each day over the last 4 weeks.

This is a pretty easy change to make, just wanted to make a note, because I think it would be a good improvement to get in before 0.15.

@sdaftuar Any thoughts on above?

After sitting on this idea for a week, I realized this morning it has a pretty big flaw. The whole point of the quick decay was to prevent mempool stuffing attacks causing artificially high fees for an extended period of time. This idea would break that.

But do you think there is some middle ground? Or maybe it is ok if it's only used for conservative estimates?

After further discussion. It's not clear how big a problem this currently is or whether a solution along these lines would even help. (For instance, do non-public nodes routinely see accurate mempool min fees or are they just never even receiving the transactions due to their peers mempool limiting?)

Closing for now and maybe forever, you can remove the milestone.