Kraken: Fix sending trades to the websocket DataHandler (#1813)

* Send trades down the DataHandler

When the tradeFeed is enabled, send the trades down the DataHandler
Add TestWSProcessTrades

* Update assertions

* Add check against null references in slices

* Add an error for ws parsing field to common

* Update kraken websocket ProcessTrades

Send individual trades down the DataHandler
Send multiple trades in test
Test error if the trade length is too short
Nits

* Fix nits

testdata/configtest.json

@@ -1968,7 +1968,9 @@
     },
     "enabled": {
      "autoPairUpdates": true,
-     "websocketAPI": true
+     "websocketAPI": true,
+     "saveTradeData": false,
+     "tradeFeed": true
     }
    },
    "bankAccounts": [