RpcServerSettings should use new BigDecimal decimal constructor instead of Parse

[devhawk]

converting decimal to BigDecimal by converting to string and then parsing is inefficient and subject to globalization errors like #466. The RpcServerSettings ctor should use the new decimal constructor (neo-project/neo#2246) once it's available

[chenzhitong]

string input = "1,234.01";
var a = (long)BigDecimal.Parse(input, NativeContract.GAS.Decimals).Value;
var b = new BigDecimal(Convert.ToDecimal(input));

It seems that we can't use new BigDecimal instead of BigDecimal.Parse
because the two values are not equal.

You see, whether this PR neo-project/neo#2248 is useful?

[devhawk]

@chenzhitong I'm suggesting that we don't use BigDecimal.Parse to convert maxGasInvoke and maxFee decimal values to BigDecimal in this code:

neo-modules/src/RpcServer/RpcServerSettings.cs

Lines 51 to 52 in fd52f21

	this.MaxGasInvoke = (long)BigDecimal.Parse(maxGasInvoke.ToString(CultureInfo.InvariantCulture), NativeContract.GAS.Decimals).Value;
	this.MaxFee = (long)BigDecimal.Parse(maxFee.ToString(CultureInfo.InvariantCulture), NativeContract.GAS.Decimals).Value;