eth/gasprice: set default percentile to 60%, count blocks instead of transactions (#15828)

The first should address a long term issue where we recommend a gas
price that is greater than that required for 50% of transactions in
recent blocks, which can lead to gas price inflation as people take
this figure and add a margin to it, resulting in a positive feedback
loop.

eth/config.go

@@ -49,8 +49,8 @@ var DefaultConfig = Config{
 
 	TxPool: core.DefaultTxPoolConfig,
 	GPO: gasprice.Config{
-		Blocks:     10,
-		Percentile: 50,
+		Blocks:     20,
+		Percentile: 60,
 	},
 }