simplify logic for history based pruning

[vondele]

Use -1 one as a sentinel value, which allows for simplifying the logic for pruning later.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 34255 W: 6292 L: 6194 D: 21769

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 89914 W: 11769 L: 11739 D: 66406

Bench: 5767899

[zamar]

Problems with this patch:

1. std::memset(-1, ...) is undefined behaviour based on standard.

2. If anyone will try to tweak VALUE_ZERO downwards (which some will do sooner or later), they will be break the implementation.

[vondele]

@zamar this would be a version that addresses both concerns. What do you think ?

[zamar]

That looks fine for me. However you should add a comment about the meaning of cmThreshold (search.cpp, line 78)