fix: make Mixologist async-safe, deprecate ObjectAggregator

[irfanuddinahmad]

Summary

• Removes threading.RLock (_CLASS_CACHE_LOCK) from Mixologist's class cache, making mix() safe for ASGI/async deployments. dict.setdefault() atomicity (via CPython's GIL) provides equivalent thread safety without ever blocking the event loop.
• Deprecates ObjectAggregator with a DeprecationWarning — it has no production callers anywhere in XBlock or edx-platform and will be removed in a future major release.

Why the lock had to go

In ASGI (single event-loop thread), calling with threading.RLock(): stalls the entire event loop if another operation holds the lock, blocking every concurrent request. This is one of the threading hazards catalogued in the parent epic.

Two reasons no replacement lock is needed:

1. asyncio is cooperative — mix() has zero await points, so the scheduler can never interleave two calls on the same thread.
2. CPython's GIL makes dict.setdefault() atomic — under true multi-threading, two racing threads may both construct the type(...) object, but setdefault guarantees only one is stored. The other is discarded. This is a benign, one-time cost.

Test plan

• All existing TestMixologist tests pass unchanged (caching and mixin behaviour is identical)
• TestObjectAggregator tests pass and assert the new DeprecationWarning fires on instantiation
• grep threading xblock/runtime.py returns nothing

Closes #918
Part of openedx/openedx-platform#38680

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