Tiered compaction: add order asserts after delta key k-merge

[arpad-m]

Adds ordering asserts to the output of the delta key iterator MergeDeltaKeys that implements a k-merge.

Part of #7296 : the asserts added by this PR get hit in the reproducers of #7296 as well, but they are earlier in the pipeline.

[github-actions]

3024 tests run: 2891 passed, 0 failed, 133 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 31.2% (6257 of 20054 functions)
• lines: 46.7% (46938 of 100592 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a3e222d at 2024-05-08T02:02:09.423Z :recycle:}