GCS artifact storage should be async

At this moment GCS artifact storage implementation relies on GCS client which does not support asyncio.

This results in the event loop being blocked on every I/O operation, which is critical for production use.

I believe this can be solved in a more generic way with S3 compatible interface (e.g. #555).
But as a first iteration, we can simply run the current methods with a thread pool executor to satisfy the minimal async behavior.