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

engine/sync_manager.go

@@ -621,7 +621,10 @@ func (m *syncManager) worker() {
 												if m.config.Verbose {
 													log.Debugf(log.SyncMgr, "Initialising %s REST ticker batching", exchangeName)
 												}
-												result, err = exchanges[x].UpdateTicker(c.Pair, c.AssetType)
+												err = exchanges[x].UpdateTickers(c.AssetType)
+												if err == nil {
+													result, err = exchanges[x].FetchTicker(c.Pair, c.AssetType)
+												}
 												m.tickerBatchLastRequested[exchangeName] = time.Now()
 												m.mux.Unlock()
 											} else {