layer file download: final rename: fix durability

[problame]

Before this PR, the layer file download code would fsync the inode after
rename instead of the timeline directory. That is not in line with what
a comment further up says we're doing, and it's obviously not achieving
the goal of making the rename durable.

part of #6663

[koivunej]

Regardless of fsync(new) after rename or the discrepency due to lack of it to the comment above, this is already a good fix. Good note on the sync_all blocking on std-fs, bifurcation.

[github-actions]

2484 tests run: 2362 passed, 0 failed, 122 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 28.8% (6960 of 24202 functions)
• lines: 47.2% (42625 of 90217 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
cc22d64 at 2024-03-05T11:15:22.517Z :recycle:}