* 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: Setup exchange manager earlier
So it's available for applications before starting.
* engine: Add a custom exchange builder interface
Allows applications that import GCT as a lib to build new
custom exchanges without further modifications.
