Multithreaded search.

[mateusz2173]

We want to enable simple multithreaded search as follows:

• Add engine option which can be set by setoption command like this: setoption name Threads value <int>
  which will set desired number of threads (1 by default)

Let's say N = number of threads

• We run 1 main thread and N-1 additional threads
• Both main and additional threads are running search on same position and same depth.
• Both main and additional threads should use same, shared transposition table
• Additional threads DO NOT output anything to stdout (they should not output any info or bestmove etc)
• We ignore any output from additional threads.

The main reason we do that is additional threads will populate transposition table making main thread search quicker.

This is not ideal utilization of multiple threads but it is good enough for us for now.

[mateusz2173]

Should be done after #17 and #19

[DomiKoPL]

Is this called Lazy SMP? https://www.chessprogramming.org/Lazy_SMP

[Gustav2137]

Lazy SMP has different depths for different threads.

[mateusz2173]

It's almost the same.

LazySMP afaik runs from different depths to enforce more "randomness" in populating TT

We could have lazysmp immediately if anyone decides what I described in original post is too easy :P i believe lazysmp might be better from what I suggested

[DomiKoPL]

I'm fine with a simpler solution. It would be even better to actually implement it first and then benchmark how much we gained from "real" LazySMP