[core] MarketDataService ::getTicker(Instrument) should be restored with the default 5.0.11 implementation

[opcoach]

From 5.0.12 the default interface implementation of MarketDataService::getTicker(Instrument ...) has changed like this (left : master, right 5.0.11) :

So, if you use the Bitstamp exchange from 5.0.12 and call the getTicker(Instrument ), we get an NotYetImplementedException for getTicker(Instrument...)... because it is not implemented in BitstampMarketDataService(/Raw)

Actually the default implementation proposed in MarketDataService (from 5.0.12) for all exchanges will be correct only when all the different exchanges have implemented the getTicker(Instrument ...).

So restoring the default implementation of 5.0.11 could be nice...