refactor: rename async_run to background_run & sync_run to main_run

[hardfist]

Summary

sync_run is actually a async function now, so the sync_run name is kind of awkward, what async_run and sync_run actually means is

• sync_run: can only be running in main thread(the thread drives task_loop), cause it can exclusively access mutable global state
• async_run: can be running in background threads
  so I think run_in_main_thread and run_in_backgrund_thread is more accurate, but it's too verbose
  so change sync_run to main_run and async_run to background_run

I'm open to better name cause I don't think these two names are perfect

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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