24-1: Fix read iterator local snapshot consistency. Fixes #2885. #3074
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A very rare failure was detected with Jepsen, where an immediate (but locally upgraded to repeatable) read caused dependency confusion between concurrent writes, making it possible to commit writes to the same key in the reverse mvcc version order. Additional issues have been identified, which made it possible for local snapshot reads to violate repeatable read guarantees, those have also been fixed with corresponding regression tests.
Fixes #2885.
Merges #3037.