CryptoExchange.Net Version 10.6.0

Updated symbol tracking logic on UserDataTracker, now is per UserDataTracker instead of per topic
Added check for startTime filter for polling being to close to current time which can cause issues if time isn't in sync with server
Added AddTrackedSymbolsAsync and RemoveTrackedSymbolAsync methods to UserDataTracker
Added check SymbolOrderBook is still alive when trying to add updates to prevent unnoticed growing in the background when subscription isn't closed while book is