fix(pageserver): only deserialize reldir key once during get_db_size

[skyzh]

Problem

fix #12101; this is a quick hack and we need better API in the future.

In get_db_size, we call get_reldir_size for every relation. However, we do the same deserializing the reldir directory thing for every relation. This creates huge CPU overhead.

Summary of changes

Get and deserialize the reldir v1 key once and use it across all get_rel_size requests.

[github-actions]

8525 tests run: 7942 passed, 0 failed, 583 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_s3_wal_replay[local_fs]: release-x86-64-without-lfc

Postgres 14

• test_sharding_split_failures[failure13]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.2% (9046 of 28062 functions)
• lines: 48.6% (79973 of 164688 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
ec80d90 at 2025-06-03T05:12:34.510Z :recycle:}