Farmer cleanups

[nazar-pc]

A few things extracted from local branch.

Plotter refactoring is the same code as before, just the way semaphore is acquired is different in try_plot_sector.

Not waiting for plotted pieces is important because in farming cluster case it will be re-initialized many times as farms come and go, which wasn't a concern previously. It is still rare, so try_read() will succeed in most cases. More sophisticated approach would be to wait until some timeout, someone is free to implement that if they care about it enough.

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