Writes to the minValid document need to be timestamped so that they don't persist after a call to recoverToStableTimestamp. During secondary batch application there are 2 main writes to the minvalid collection that are relevant: one that sets the minValid to the end of the batch and another that sets the appliedThrough to the end of the batch. Both those writes should be given the timestamp of the end of the batch, so that if we recover to a time before the batch the writes will be undone.