Poloniex ccxtpro

Added watchTrades and watchTickers using cctx.pro methods.

Please check if implemented correctly.

Left not used 'handleMessage' method for a reference. Delete if new subscription methods passing data to this.subscribedSymbols correctly, or adjust accordingly.

Note. For this to run minimum version of ccxt is 1.95+ when pro functionalities has been added. Implemented and tested with newest version "ccxt": "^4.4.2"