code: public Closeable onExecutedTradeEvent(final BiConsumer<BitfinexExecutedTradeSymbol, Collection<BitfinexExecutedTrade>> listener) {
          executedTradesConsumers.offer(listener);
          return () -> executedTradesConsumers.remove(listener);
      }
docs: registers listener for general trades executed within scope of exchange instrument (ie. tBTCUSD)
  @param listener of event
  @return hook of this listener