Auto merge of #22769 - gterzian:use_thread_pool_in_resource_thread, r=nox

Use a threadpool for fetch

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #22768 (GitHub issue number if applicable)

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Cargo.lock

@@ -2805,6 +2805,7 @@ dependencies = [
  "openssl 0.10.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "pixels 0.0.1",
  "profile_traits 0.0.1",
+ "rayon 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 1.0.80 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_json 1.0.13 (registry+https://github.com/rust-lang/crates.io-index)",
  "servo_allocator 0.0.1",