rt: track loom changes + tweak queue (#2315)

Loom is having a big refresh to improve performance and tighten up the
concurrency model. This diff tracks those changes.

Included in the changes is the removal of `CausalCell` deferred checks.
This is due to it technically being undefined behavior in the C++11
memory model. To address this, the work-stealing queue is updated to
avoid needing this behavior. This is done by limiting the queue to have
one concurrent stealer.

ci/azure-test-stable.yml

@@ -25,7 +25,7 @@ jobs:
     # Run with all crate features
     - script: cargo test --all-features
       env:
-        LOOM_MAX_PREEMPTIONS: 2
+        RUST_BACKTRACE: 1
         CI: 'True'
       displayName: ${{ crate }} - cargo test --all-features
       workingDirectory: $(Build.SourcesDirectory)/${{ crate }}
@@ -41,7 +41,7 @@ jobs:
     # Run with all crate features
     - script: cargo test --all-features
       env:
-        LOOM_MAX_PREEMPTIONS: 2
+        RUST_BACKTRACE: 1
         CI: 'True'
       displayName: ${{ crate }} - cargo test --all-features
       workingDirectory: $(Build.SourcesDirectory)/${{ crate }}