[Transform] Odd logging of version conflict on stop in rare cases

[hendrikmuhs]

Followup from: #51629

We use optimistic concurrency control and keep the seq_nr/primary term in an atomic variable, still, when stopping a transform while at the same time it auto-stops, it is possible that 2 save state calls run at the same time from different threads. This is usually only the case if transforms is automated, e.g. this was found via CI.

The issue is benign, no information gets lost. Still it produces these warnings in the log, which is a bad user experience:

01:44:49 »  Caused by: org.elasticsearch.index.engine.VersionConflictEngineException: [data_frame_transform_state_and_stats-simple-local-remote-transform]: version conflict, document already exists (current version [1])
01:44:49 »  	at org.elasticsearch.index.engine.InternalEngine.planIndexingAsPrimary(InternalEngine.java:1062) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.index.engine.InternalEngine.indexingStrategyForOperation(InternalEngine.java:1021) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.index.engine.InternalEngine.index(InternalEngine.java:910) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.index.shard.IndexShard.index(IndexShard.java:813) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.index.shard.IndexShard.applyIndexOperation(IndexShard.java:785) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.index.shard.IndexShard.applyIndexOperationOnPrimary(IndexShard.java:742) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.action.bulk.TransportShardBulkAction.executeBulkItemRequest(TransportShardBulkAction.java:254) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	at org.elasticsearch.action.bulk.TransportShardBulkAction$2.doRun(TransportShardBulkAction.java:157) ~[elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
01:44:49 »  	... 148 more
01:44:49 » WARN ][o.e.p.AllocatedPersistentTask] [mixed-cluster-0] attempt to complete task [data_frame/transforms[c]] with id [simple-local-remote-transform] in the [COMPLETED] state

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)