Context

We have pieces of code that take in a mongo.SessionContext and are to be run within a mongo transaction. The only real way to determine that today is to call StartTransaction() and check for an ErrTransactionInProgress, which is part of the /x/ package and not a stable API. 

Wanted to propose expose a method as part of the stable API for identifying whether a mongo.SessionContext is running within a transaction or not. 

Definition of done

The driver exposes a method on a mongo.Session that returns a boolean representing whether it is running within a transaction

Pitfalls