Back when we had atomic applyOps, we had an "alwaysUpsert" field in applyOps which would tell secondaries whether a 'u' operation should be treated as an upsert. Confusingly, this field defaults to "true" for applyOps operations without a transaction number. We haven't had atomic applyOps since 6.2, so we should be able to remove support for alwaysUpsert and treat it as always false.

This will make things easier if we support batching of updates in applyOps entries; right now such updates will be treated as upserts, which, while it should not result in incorrect operation, bypasses checks for consistency.