gh-128002: use per threads tasks linked list in asyncio

[kumaraditya303]

• Issue: Audit asyncio thread safety  #128002

Use per-thread linked list of tasks in asyncio. This design allows for lock free register/unregister of tasks of loops running concurrently in different threads. It uses the stop the world pause to traverse the list of tasks from all threads from the thread where all_tasks is called. This has no performance impact on regular builds as per benchmarks and performs a bit faster on free-threading benchmarks. pyperformance benchmarks aren't good for this because it uses just one thread so there is little lock contention, this however performs much better when multiple threads are running.

On free-threading:

Benchmark	bm-20250111-linux-x86_64-python-3a570c6d58bd5ad7d7c1-3.14.0a3+-3a570c6	bm-20250112-linux-x86_64-kumaraditya303-per_thread_tasks-3.14.0a3+-cca4057
async_tree_cpu_io_mixed_tg	556 ms	536 ms: 1.04x faster
async_tree_none_tg	303 ms	294 ms: 1.03x faster
coroutines	26.2 ms	25.5 ms: 1.03x faster
async_tree_memoization_tg	397 ms	387 ms: 1.03x faster
async_tree_cpu_io_mixed	598 ms	583 ms: 1.03x faster
async_generators	498 ms	486 ms: 1.02x faster
async_tree_io	748 ms	733 ms: 1.02x faster
async_tree_io_tg	696 ms	682 ms: 1.02x faster
async_tree_memoization	442 ms	436 ms: 1.02x faster
async_tree_none	349 ms	344 ms: 1.01x faster
Geometric mean	(ref)	1.02x faster

Benchmark hidden because not significant (1): asyncio_websockets

[1st1]

First, great work on this, this is legitimately a cool PR.

That said, I'm feeling really uneasy about _PyEval_StopTheWorld. Maybe instead of this approach we try spin locks + a custom mini hash map data structure? That would obviously be a lot more work, but a cleaner and ultimately better perf approach.

Also please wait for reviews from @pablogsal and @ambv. I'm curious if this would make external introspection harder.