IID margin reduced to 128

IID margin reduced to 128

Change the internal iterative deepening margin from +256 to +128.

Passed STC:
LLR: 3.26 (-2.94,2.94) [0.00,5.00]
Total: 22006 W: 4646 L: 4400 D: 12960
http://tests.stockfishchess.org/tests/view/5ab54a240ebc590295d8847d

Passed LTC:
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 46665 W: 7187 L: 6898 D: 32580
http://tests.stockfishchess.org/tests/view/5ab5ba690ebc590295d884b4

Possible work in this area after this change:

• Reduce even more the margin?
• Try to do IID for smaller depths too?

Bench: 5851102