Market data subscription

After connecting to the market data channel, the client subscribes to market data by sending a Market Data Request <V> message.

When a client disconnects for any reason, the market data subscription is terminated. Upon reconnecting, the client needs to send another Market Data Request <V> message.

1. Client sends a Market Data Request <V>
2. Gemini responds by sending one Market Data - Snapshot/Full Refresh <W>
3. As bids, offers, and trades happen on the exchange, Gemini sends Market Data - Incremental Refresh <X> messages