Dynamic gas price oracle to reduce dusk attack possibility

[DarianShawn]

Currently, gas price oracle will give out suggest price based on historical price. It is OK in most time, but not working good if the network is facing dusk attack.

Most of the network users were struggling with not being able to have their transactions included when network is being attacked, and they can't find out why and how to pass it through.

The client, especially the RPC endpoint, has the responsibility and ability to suggest higher gas price to users so as to have their transactions included as soon as possible.

But we have to take care that raising price will make all network users cost more in network congestion whether they wish it or not. The most important part is, validator nodes are the final guard who are responsible to protect dust attack, and they should be able to give out a reasonable suggested gas price, which is lower than other RPC endpoints (more pressure), but high enough for reducing attackers' willingness to attack.

[DarianShawn]

This call is still in draft.
More suggestions are welcome.
@abrahamcruise321 @0xcb9ff9

[DarianShawn]

Gas and number of transactions in the txpool should be considered, especially when we evaluate whether the current network is congested.
I think it is necessary to use some filter functions to make the prediction more precisely.
And yes, I suppose that we need to do some prediction since the gas price in original GPO implementation is statistic data.
Sometimes it will just overfit or underfit if we use the median simply. Using prediction data will make the trend more smoothly, and so as the balance between transaction fee and including speed.