6444: Ingest batcher discards files if a job fails to send

[ac28474]

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Issue

• My PR fully resolves the following issues. I've referenced an issue in the PR title, for example "Issue 1234 - My
  Feature". Note that before an issue is finished, you can still make a pull request by raising a separate issue
  for your progress.
  • Ingest batcher discards files if a job fails to send #6444

Tests

• My PR amends the following tests based on our test strategy OR does not need testing for this extremely good reason:
  • IngestBatcherIngestModesTest.java
  • IngestBatcherUpdateStoreTest.java
  • InMemoryIngestBatcherStoreTest.java

Documentation

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  separate issue for that below.
• If I have added new Java code, I have added Javadoc that explains it following our conventions and style.
• If I have added or removed any dependencies from the project, I have updated the NOTICES file.