Currently in 2.9.0-beta1, the pinned server (for mongos pinning) is stored in as a public property in the session object. It may make more sense to store it within the CoreTransaction object because a new CoreTransaction is generated for each transaction, saving us the trouble of resetting the pinned server in the session to null. If we make this change, it would have to before the final release of 2.9.0 as it would otherwise be a breaking change to the ICoreSession api.