Currently if we fail to parse an oplog entry when trying to check out a session, that command will fail as will all subsequent checkouts of the session. This means that on downgrade, sessions referencing oplog entries from future versions may become unusable and the user/driver will have to discard them.

Instead, failure to parse should leave the session in the same state as if the oplog entry could not be found (incomplete transaction history), meaning retries of pre-downgrade transactions will fail and newer transactions will succeed.