Threading Issue for Subscription Manager

[colinkimball]

The subscription manager polls each exchanges' REST API periodically to receive a fresh listing of supported markets as well as the best bid/ask orders for each market. When it receives this information, and markets have been added/delisted, it updates the 'TriarbMarketMapping' concurrent dictionary for its respective exchange.

For a widely referenced market such as BTC/USDT, we occasionally encounter an exception because the OrderbookListener is attempting to iterate over the Triangles mapped to the symbol while the SubscriptionManager is modifying that list at the same time.