Add FullAccessTimeline guard in safekeepers

[petuhovskiy]

This is a preparation for #6337.

The idea is to add FullAccessTimeline, which will act as a guard for tasks requiring access to WAL files. Eviction will be blocked on these tasks and WAL won't be deleted from disk until there is at least one active FullAccessTimeline.

To get FullAccessTimeline, tasks call tli.full_access_guard().await?. After eviction is implemented, this function will be responsible for downloading missing WAL file and waiting until the download finishes.

This commit also contains other small refactorings:

• Separate get_tenant_dir and get_timeline_dir functions for building a local path. This is useful for looking at usages and finding tasks requiring access to local filesystem.
• timeline_manager is now responsible for spawning all background tasks
• WAL removal task is now spawned instantly after horizon is updated

[github-actions]

3156 tests run: 3017 passed, 0 failed, 139 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Postgres 15

• test_timeline_deletion_with_files_stuck_in_upload_queue: debug

Code coverage* (full report)

• functions: 31.4% (6494 of 20689 functions)
• lines: 48.4% (50232 of 103835 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
07a3e9a at 2024-05-31T13:28:38.434Z :recycle:}