Move some code into threads

[protonspring]

This is a non-functional code style change that moves some pure thread code into the threads class.

It is a bit more code, but it makes search.cpp cleaner and easier to read by hiding some thread specific functionality.

STC (SMP)
LLR: 2.93 (-2.94,2.94) {-1.50,0.50}
Total: 75896 W: 12073 L: 12026 D: 51797
Ptnml(0-2): 828, 8224, 19872, 8121, 903
https://tests.stockfishchess.org/tests/view/5ed492e8f29b40b0fc95a74c

[vondele]

I'm wondering about this th->bestMoveChanges = 0; statement. It is in the same location as master, but it is not quite logical. Can't it be moved to e.g. Thread::search() or ThreadPool::start_thinking() (the latter place seems most logical for anything that is an atomic in Thread).

[xoto10]

Should we do a threaded non-regression test? I don't expect it to be a problem, but it would be embarrassing if a threading code tidy up caused a regression.

[vondele]

@xoto10 the test was threaded, it is not clear from the commit message.

[vondele]

@protonspring can you have a look at the bestMoveChanges I made before. Now that the code is refactored, it is a good moment to fix it.

[protonspring]

I haven't looked much here, but on first glance, I would set bestMoveChanges to 0 on search(EDIT or start_thinking as suggested above), and add another method for ThreadPool::get_bestMoveChanges which gets the total for all threads in the pool. This would further simplify search.

[vondele]

Thanks!