Async wrapper for FastAPI endpoints to run serialization in event loop

[schustmi]

Describe changes

When having a sync FastAPI endpoint, it runs the endpoint function in a worker threadpool. If all threads are busy, it will queue the task. The problem is that after the endpoint code returns, FastAPI will queue another task in the same threadpool to serialize the response. If there are many tasks already in the queue, this means that the response serialization will wait for a long time instead of returning the response immediately. By making our endpoints async and then immediately dispatching them to the threadpool ourselves (which is essentially what FastAPI does when having a sync endpoint), we can avoid this problem. The serialization logic will now run on the event loop and not wait for a worker thread to become available.

See: https://github.com/fastapi/fastapi/blob/ea7b1054762c72a79bc111e747e20d4c67721afc/fastapi/routing.py#L165-L170 and fastapi/fastapi#888 for more information.

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Types of changes

• 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)
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