pageserver: limit number of upload queue tasks

[erikgrinaker]

Problem

The upload queue can currently schedule an arbitrary number of tasks. This can both spawn an unbounded number of Tokio tasks, and also significantly slow down upload queue scheduling as it's quadratic in number of operations.

Touches #10096.

Summary of changes

Limit the number of inprogress tasks to the remote storage upload concurrency. While this concurrency limit is shared across all tenants, there's certainly no point in scheduling more than this -- we could even consider setting the limit lower, but don't for now to avoid artificially constraining tenants.

[github-actions]

7304 tests run: 6933 passed, 0 failed, 371 skipped (full report)

---

Flaky tests (5)

Postgres 17

• test_nbtree_pagesplit_cycleid: release-arm64, release-arm64
• test_storage_controller_node_deletion[True]: debug-x86-64

Postgres 15

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64

Postgres 14

• test_timeline_archive[4]: release-arm64

Code coverage* (full report)

• functions: 32.9% (8145 of 24776 functions)
• lines: 48.1% (68239 of 141903 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
fcbe2e8 at 2025-01-14T18:01:32.529Z :recycle:}