Smart TT save
Don't overwrite more valuable data with less valuable data
STC 2MB
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 21132 W: 4108 L: 3946 D: 13078
http://tests.stockfishchess.org/tests/view/5547d59f0ebc5940ca5d6883
LTC 8MB
LLR: 2.97 (-2.94,2.94) [0.00,6.00]
Total: 13381 W: 2149 L: 1987 D: 9245
http://tests.stockfishchess.org/tests/view/5549b5a80ebc5940ca5d68b9
STC 16MB regression w/ zero effective hash pressure
LLR: 2.96 (-2.94,2.94) [-5.00,0.00]
Total: 18944 W: 3607 L: 3564 D: 11773
http://tests.stockfishchess.org/tests/view/554b0fda0ebc5940ca5d68ea
Bench: 8787152
Resolves #347