As part of the SBE yielding work, we'll be stashing the RecoveryUnit on ClientCursors for find, aggregate and getMore commands.

There is already similar machinery for multi-document transactions, which stores the RecoveryUnit across multiple operations (not just find()/aggregate()/getMore). This is built on the TransactionParticipant and related classes.

This task is to investigate and attempt to unify these two functionalities. We may find that the transaction machinery is "too heavyweight" for what we want, or it may end up being a good simplification of the codebase.