WT-6421Avoid parsing metadata checkpoint for clean files.
- Set timer to forever if we know we'll never have to delete older
- Fix WT_RET to WT_ERR.
- Remove unneeded WT_UNUSED. And formatting.
- Add per-btree stat for clean checkpoint timer
- Add new stat to list we cannot detect.
- Add test and fixes for it.
- Clean up and simplify test
- Fix test to check values are within 1 second of each other.
- Style fixes. Remove unneeded code/comment. | 06 Aug 20 18:17 UTC
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