Refund extra GAS

[Jim8y]

This is an experimental work trying to study the possiblity of refunding extra system fee user pays.

Solution here is only burn the amount of consumed system fee instead.

Open to comments and suggestations.

Gas consume table after analysing 1,291,140 transactions in neo N3:

10%	20%	30%	40%	50%	60%	70%	80%	90%	100%
32211	37606	49009	12775	27212	9822	4861	15839	257764	844041
2.49%	2.91%	3.8%	0.99%	2.11%	0.79%	0.38%	1.23%	19.96%	65.37%

From the table we can see that around 35% transactions pay more GAS fee than needed, and 2.49% transaction even pay 10 times more transaction fee than needed.