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

exchanges/sharedtestvalues/customex.go

@@ -51,6 +51,10 @@ func (c *CustomEx) FetchTicker(p currency.Pair, a asset.Item) (*ticker.Price, er
 	return nil, nil
 }
 
+func (c *CustomEx) UpdateTickers(a asset.Item) error {
+	return nil
+}
+
 func (c *CustomEx) UpdateTicker(p currency.Pair, a asset.Item) (*ticker.Price, error) {
 	return nil, nil
 }