Fix plotting getting stuck

[nazar-pc]

This is essentially a regression from #3160.

I'm not really sure under which circumstances, but it is hypothetically possible for sectors_metadata.read().await in plot_single_sector to yield to executor after already reserving the RwLock acquisition slot, such that when sectors_metadata.write().await is called in process_plotting_result after #3160 just hangs forever.

Essentially an issue of a similar nature to #3041, which is counter-intuitive because it behaves differently from synchronous counterparts in standard library and parking_lot.

The solution is to allow these to run process_plotting_result concurrently with everything else, which also has added benefit of making some progress for plotting elsewhere while writes in process_plotting_result are happening.

This is a less hacky version that should help avoiding similar issues in the future if/when we do further refactoring.

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[teor2345]

Thanks for tracking this down, looks annoying!