Depth dependant aspiration window delta

Split delta value in aspiration window so that when
search depth is less than 24 a smaller delta value
is used. The idea is that the search is likely to
be more accurate at lower depths and so we can exclude
more possibilities, 25% to be exact.

Passed STC
LLR: 2.96 (-2.94, 2.94) [-1.50, 4.50]
Total: 20430 W: 3775 L: 3618 D: 13037

And LTC
LLR: 2.96 (-2.94, 2.94) [0.00, 6.00]
Total: 5032 W: 839 L: 715 D: 3478

Bench: 7451319
Microptimize castling in undo_move()

We don't need to set 'captured' and 'pt' after we
castle back.

No functional change.
Merge default tests in pos_is_ok

No functional change.
Remove "Max Threads per Split Point" UCI option

Experimental patch to verify if drop of nps
in endgames at very long TC is due to this.

Suggested by Ronald de Man.

bench: 7451319
Split also if no slaves are found

Because we test for available slaves before
entering split(), we almost always allocate a
slave, only in the rare case of a race (less
then 2% of cases) this is not true, but to
special case this occurrence is not worth
the added complexity.

bench: 7451319
Fix a crash under MSVC

Using memset on a std::vector is undefined behavior,
so manually init all the data memebers of LimitsType.

Bug intorduced in 41641e3b1eea0038ab6984

No functional change.