Put get-released-data record-count and result-stream in one transaction

[corneliusroemer]

Low priority

See:

loculus/backend/src/main/kotlin/org/loculus/backend/controller/SubmissionController.kt

Lines 272 to 275 in bc59479

	// There's a possibility that the totalRecords change between the count and the actual query
	// this is not too bad, if the client ends up with a few more records than expected
	// We just need to make sure the etag used is from before the count
	// Alternatively, we could read once to file while counting and then stream the file

This was discussed in PR, @anna-parker tested (IIUC) that this can happen (by introducing an exaggeratedly long delay between count and streaming through sleep - it is unlikely to cause any harm in practice, and the PR makes things in no way worse than without it.

#2773

[chaoran-chen]

As this means that the value in the header can potentially be wrong, I'd rather classify it as a bug. If there are few sequences and unfrequent new submissions, I agree that it is not a big problem. If we go to an outbreak / pandemic situtation with millions of sequences and frequent uploads, this changes.