Issue 6471 - When two bulk import jobs extend the partition tree at once, one fails

[patchwork01]

Make sure you have checked all steps below.

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.
  • Resolves When two bulk import jobs extend the partition tree at once, one fails #6471

Tests

• My PR adds the following tests based on our test strategy OR does not need testing for this extremely good reason:
  • PartitionPreSplitterTest
  • Updated BulkImportJobDriverTest

Documentation

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  separate issue for that below.
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[patchwork01]

It doesn't look like we're testing that the pre-splitter can actually perform a retry. Here are some potential tests:

• We want to split from 1 to 4 partitions. On the first attempt, it conflicts as another process splits to 2 partitions. On the second attempt, it regenerates the sketches for the new leaf partitions and successfully splits to 4 partitions.
• We want to split from 1 to 8 partitions. We allow 2 attempts. On the first attempt, it conflicts as another process splits to 2 partitions. On the second attempt, it conflicts as another process splits to 4 partitions. It fails because it runs out of attempts.