Track information about last client to run a transaction operation on session

To support the currentOp output for inactive transactions, we want to keep track of information about the last client to run an operation against a particular session. Normally, a single operation in currentOp (not a transaction) is only associated with a single client. For sessions (and transactions), though, it is possible that different clients run operations against a single session. Often, all operations will be run by the same client, and so it would be helpful to store this information on the session, so we can report it in currentOp. The information we hope to store about the last client is connectionId, client, appName, and clientMetadata.