Kraken: speedup symbol lookup in trade stream (#473)

* Kraken: speedup symbol lookup in trade stream

- Kraken uses special symbols (ADA/CAD rather than ADACAD)
- a recent bugfix fa2534e now sets the WS name as an alternate name
- this ExchangeMarket is looked up for each symbol name to pull out the alternate name
- while it attempts to use caching, it is not actually able to use the cache, bc once it hits the await MakeJsonRequestAsync() in OnGetMarketSymbolsMetadataAsync(), the next select statement progresses so OnGetMarketSymbolsMetadataAsync() is called for every symbol
- by priming the cache first, this allows the cache to populate first before the select statement, thus hitting OnGetMarketSymbolsMetadataAsync() once

* change to PopulateLookupTables() for priming

Author: vslee

ExchangeSharp/API/Exchanges/Kraken/ExchangeKrakenAPI.cs

@@ -859,16 +859,17 @@ protected override async Task<IWebSocket> OnGetTradesWebSocketAsync(Func<KeyValu
 				}
 			}, connectCallback: async (_socket) =>
 			{
-                //{
-                //  "event": "subscribe",
-                //  "pair": [
-                //    "XBT/USD","XBT/EUR"
-                //  ],
-                //  "subscription": {
-                //    "name": "ticker"
-                //  }
-                //}
-                Task<string>[] marketSymbolsArray = marketSymbols.Select(async (m) =>
+				//{
+				//  "event": "subscribe",
+				//  "pair": [
+				//    "XBT/USD","XBT/EUR"
+				//  ],
+				//  "subscription": {
+				//    "name": "ticker"
+				//  }
+				//}
+				await PopulateLookupTables(); // prime cache
+				Task<string>[] marketSymbolsArray = marketSymbols.Select(async (m) =>
                 {
                     ExchangeMarket market = await GetExchangeMarketFromCacheAsync(m);
                     if (market == null)