AJ-1498: tracing and isolation level for updateStatisticsCache

[davidangb]

Ticket: https://broadworkbench.atlassian.net/browse/AJ-1498

More optimizations and debuggability for the entity statistics cache.

• explicitly set the transactions in updateStatisticsCache to be ReadCommitted. This is probably moot, since this method is only called from sweep(), and we set the outer transaction in sweep() to ReadCommitted in AJ-1498: reduce transaction isolation level for entity metadata #2650. But, it helps for code readability and future-proofing.
• move the declaration of ReadCommitted to a trait-level variable for consistency
• add more detail to the tracing spans under sweep(). This is likely to confirm previous knowledge that getAttrNamesAndEntityTypes is slow, but it'll be good to see empirical data.

Reviewer: I suggest ignoring whitespace in the diff; auto-formatter changed a bunch

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• Include the JIRA issue number in the PR description and title
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[davidangb]

one failure in org.broadinstitute.dsde.test.api.WorkspaceApiSpec.Rawls should allow project owners to create, clone, and delete workspaces with a google project not found.

Jenkins retest.

[davidangb]

fiab-start failed with a 503 from Sam. Jenkins retest.