This is meant as an alternative to using DBDirectClient, which sets the 'inDirectClient' flag on the passed-in OperationContext, and so cannot be used to run operations under a session.

handleInternalRequest will behave similarly to DBDirectClient, except will take (or create) a "fresh" OperationContext and not set the 'inDirectClient' flag on it (though it will grant internal authorization to it).

This method is essentially syntactic sugar: it should take a BSONObj request, serialize it into an OpMsgRequest, call handleRequest on the OpMsgRequest, and deserialize the response into a BSONObj.