OKEx RateLimit (#694)

Increased the rate limit to 20req/2sec. It's not the real limit, but somehow "averaged", because each endpoint has it's own limit.

Author: BZ-CO

src/ExchangeSharp/API/Exchanges/OKGroup/ExchangeOKExAPI.cs

@@ -34,6 +34,11 @@ public sealed partial class ExchangeOKExAPI : OKGroupCommon
 public string BaseUrlV5 { get; set; } = "https://www.okex.com/api/v5";
 protected override bool IsFuturesAndSwapEnabled { get; } = true;
 
+private ExchangeOKExAPI()
+{
+RateLimit = new RateGate(20, TimeSpan.FromSeconds(2));
+}
+
 public override string PeriodSecondsToString(int seconds)
 {
 return CryptoUtility.SecondsToPeriodString(seconds, true);