saveAll

Saves snapshots: in the rounds SnapshotSaveOutcome.rounds gives, each round's snapshots of one aggregate type with one unordered bulkWrite of conditional upserts, each guarded as save guards one. Each update is a pipeline that clears a stale mark of a version up to its own in the same write, so a batch that all lands is one round trip whatever marks it meets. A snapshot whose upsert met the key (the guard refused it, or a concurrent insert won the key) is saved again alone, as save would (conditionalUpsert), which also marks a higher state revision stale; one the server refused with another error, or whose state cannot be BSON, is SnapshotSaveOutcome.Failed alone.