[Threading] Threadpool, lifo semaphore

[Eideren]

PR Details

Now that .Net6 uses their fully managed threadpool by default we can hook into their lifo semaphore which improves performances slightly.
I did benchmark their System.Threading.ThreadPool.UnsafeQueueUserWorkItem again under net6 and ours still performed better, so still no reason to migrate to dotnet's threadpool.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Eideren]

Implemented through #2083