pageserver: add more info-level logging in shard splits

[jcsp]

Problem

test_sharding_autosplit is occasionally failing on warnings about shard splits taking longer than expected (Exclusive lock by ShardSplit was held for...)

It's not obvious which part is taking the time (I suspect remote storage uploads).

Example: https://neon-github-public-dev.s3.amazonaws.com/reports/main/9618788427/index.html#testresult/b395294d5bdeb783/

Summary of changes

• Since shard splits are infrequent events, we can afford to be very chatty: add a bunch of info-level logging throughout the process.

Checklist before requesting a review

• [ ] I have performed a self-review of my code.
• [ ] If it is a core feature, I have added thorough tests.
• [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• [ ] Do not forget to reformat commit message to not include the above checklist

[github-actions[bot]]

2910 tests run: 2793 passed, 0 failed, 117 skipped (full report)

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Flaky tests (1)

#### Postgres 16 - `test_tenant_delete_smoke`: [release](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8137/9636039975/index.html#suites/3556ed71f2d69272a7014df6dcb02317/86d5d8141745c902/retries)

Code coverage* (full report)

• functions: 32.4% (6869 of 21175 functions)
• lines: 49.9% (53431 of 107090 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7f7f33f7e83a4c5fdeea2353bc364428e600a607 at 2024-06-23T20:44:32.953Z :recycle:}