Fix sector plotting getting stuck

[nazar-pc]

As discovered in smol-rs/async-lock#91 (comment) original polling attempt order is important and might cause issues if code was later serialized with a different order, which is exactly what was happening in process_plotting_result on farm-specific sectors_being_modified that was causing that farm to stop plotting and I believe even farming.

The solution is to move a piece of code into concurrent section so it can make progress.

I could have removed process_plotting_result by wrapping metadata_header in a Mutex and adding even more arguments to plot_single_sector, but that would result in out of order log messages for end users and didn't seem to be worth it.

Code contributor checklist:

• I have read, understood and followed contributing guide