refactor(walredo): split up walredo.rs

[problame]

The file is massive, and can be broken up neatly into

• process lifecycle management
• IPC protocol
• applying neon-specific walredo records
• overall walredo manager

This is prep work for #6581

[github-actions]

2388 tests run: 2273 passed, 0 failed, 115 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_timeline_deletion_with_files_stuck_in_upload_queue: debug

Code coverage (full report)

• functions: 54.4% (11283 of 20741 functions)
• lines: 81.5% (63516 of 77969 lines)

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_{The comment gets automatically updated with the latest test results
05f3d70 at 2024-02-05T13:23:40.149Z :recycle:}

[VladLazar]

LGTM, but clippy has a complaint:

error: unused import: `atomic::AtomicUsize`
    --> pageserver/src/walredo/process.rs:17:12
     |
  17 |     sync::{atomic::AtomicUsize, Mutex, MutexGuard},
     |            ^^^^^^^^^^^^^^^^^^^
     |
     = note: `-D unused-imports` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(unused_imports)]`