exchanges: Add UpdateTickers method and reserve UpdateTicker for single ticker symbol update requests (if supported) (#764)

* exchanges: add an UpdateTickers method to the main exchange interface

This method will fetch all currency pair tickers of a given asset type
and update them internally, does nothing for now.

* exchanges: refactor UpdateTicker on all exchanges

Keep the exact previous behaviour but implement the UpdateTickers
method and refactor UpdateTicker by using it where applicable.

* sync_manager: update all tickers when batching is enabled

* binance: UpdateTicker to fetch single ticker symbol

* ftx: UpdateTicker to fetch single ticker symbol

testdata/http_mock/binance/binance.json

@@ -143032,6 +143032,34 @@
      "bodyParams": "",
      "headers": {}
     },
+    {
+     "data": {
+       "askPrice": "0.04103400",
+       "askQty": "1.89600000",
+       "bidPrice": "0.04103300",
+       "bidQty": "43.77800000",
+       "closeTime": 1611715404283,
+       "count": 460443,
+       "firstId": 221265272,
+       "highPrice": "0.04223600",
+       "lastId": 221725714,
+       "lastPrice": "0.04103400",
+       "lastQty": "1.06900000",
+       "lowPrice": "0.04007400",
+       "openPrice": "0.04176800",
+       "openTime": 1611629004283,
+       "prevClosePrice": "0.04176800",
+       "priceChange": "-0.00073400",
+       "priceChangePercent": "-1.757",
+       "quoteVolume": "16980.13811233",
+       "symbol": "ETHBTC",
+       "volume": "410969.51200000",
+       "weightedAvgPrice": "0.04131727"
+     },
+     "queryString": "symbol=ETHBTC",
+     "bodyParams": "",
+     "headers": {}
+    },
     {
      "data": [
       {
@@ -310430,4 +310458,4 @@
    ]
   }
  }
-}
+}