Make plotting/replotting thread pools distinct for farms to avoid stack overflow, still protected by semaphores

[nazar-pc]

Having multiple ThreadPool::install() apears to be problematic as described in rayon-rs/rayon#751

Since #2095 we have separate concurrency parameters for sector downloading and encoding, so we will not have two sectors plotting at the same time unless user wants to anyway. BTW it would open the option to have thread pool size smaller per plot and allow multiple sectors to be encoded at the same time on larger machines to not cross NUMA boundaries, but farmer would need to become NUMA-aware for that to make sense (and at that point it can do such things automatically when NUMA is detected).

Code contributor checklist:

• I have read, understood and followed contributing guide

[shamil-gadelshin]

Do I understand correctly that with the new changes and having set --sector-encoding-concurrency > 1 it's possible to undo the thread pool separation between tasks we achieved previously?

[nazar-pc]

Do I understand correctly that with the new changes and having set --sector-encoding-concurrency > 1 it's possible to undo the thread pool separation between tasks we achieved previously?

Before this PR thread pools for plotting/replotting were global, now they are deparate. Not sure what you mean by "undo" in this case.

[shamil-gadelshin]

Before this PR thread pools for plotting/replotting were global, now they are deparate. Not sure what you mean by "undo" in this case.

We can have several "single_plot_farms" simultaneously and each will create its own copies of the thread pools for both plotting and replotting, correct? This means that plotting and replotting threads could steal CPU cores from each other similar to what we had before. Does it make sense?

[nazar-pc]

This means that plotting and replotting threads could steal CPU cores from each other similar to what we had before. Does it make sense?

Not in the same way. These will be separate threads, while previously they'd compete for a shared thread pool.