unit-test-bucket07 failed on ! Ubuntu 18.04

Host: ec2-54-82-108-217.compute-1.amazonaws.com
Project: WiredTiger (develop)
Commit: diff: WT-7674 reduce rollback-to-stable work for fast-truncate pages (#6658)

• Rollback-to-stable rolls back instantiated fast-truncate pages twice, once when processing the
  parent internal page and a second time when processing the child leaf page.

I don't see a problem in the current code, but it's dangerous because rollback-to-stable uses
the WT_PAGE_DELETED.durable_timestamp value to determine if rollback is needed. When a page is
instantiated, the WT_PAGE_DELETED.durable_timestamp moves from the WT_PAGE_DELETED structure
to the individual WT_UPDATE structures, so after instantiation rollback-to-stable is taking
the value from the wrong source.

Change rollback-to-stable to only check WT_PAGE_DELETED.durable_timestamp for pages that have
not yet been instantiated.

• Checking the WT_REF.state is insufficient, WT_REF_DELETED can mean an empty page as well as
  a fast-truncate page.

• style cleanup. | 15 Jun 21 03:40 UTC
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