pageserver: rename control plane client & chunk validation requests

[jcsp]

Problem

• In storage controller: read from database in validate API #8784, the validate controller API is modified to check generations directly in the database. It batches tenants into separate queries to avoid generating a huge statement, but
• While updating this, I realized that "control_plane_client" is a kind of confusing name for the client code now that it primarily talks to the storage controller (the case of talking to the control plane will go away in a few months).

Summary of changes

• Big rename to "ControllerUpcallClient" -- this reflects the storage controller's api naming, where the paths used by the pageserver are in /upcall/
• When sending validate requests, break them up into chunks so that we avoid possible edge cases of generating any HTTP requests that require database I/O across many thousands of tenants.

This PR mixes a functional change with a refactor, but the commits are cleanly separated -- only the last commit is a functional change.

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

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[github-actions]

5013 tests run: 4849 passed, 0 failed, 164 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-arm64
• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 32.0% (7491 of 23395 functions)
• lines: 50.0% (60478 of 120871 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
0733a2e at 2024-09-26T09:22:04.082Z :recycle:}