Add BTCTradeSession to ensure the polling service instancs, which using same API key, share the same secret data and nonce, and make all requests on one API key synchronized.

If use same API key in BTCTradeAccountService and BTCTradeTradeService, will get "signature error", as they didn't share same SecretData and nonce.

But as designed, server side expects incrementing nonce and one single SecretData for one API key.

In this patch, introduce a BTCTradeSession and BTCTradeSessionFactory, to make sure we have only one instance to maintain the status of one API key.