safekeeper: check for non-consecutive writes in safekeeper.rs

[arssher]

wal_storage.rs already checks this, but since this is a quite legit scenario check it at safekeeper.rs (consensus level) as well.

ref #8212

[arssher]

There was an idea to try to differ when these conditions are 'expected' and not, but I don't see a useful way to do this. It is normal to see this in case of reconnections. Something different is hard to imagine and likely means serious bug, like wal truncation repeatedly truncating not at the point where streaming later starts; then we won't be able to write anything at all and also notice the problem.

[github-actions]

2112 tests run: 2043 passed, 0 failed, 69 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (7162 of 21950 functions)
• lines: 50.5% (57867 of 114514 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c10a29d at 2024-08-07T17:33:29.116Z :recycle:}