Restore deterministic search state

Introduce helper function Search::reset() which clears all kind of search
memory, in order to restore a deterministic search state.

Generalize TT.clear() into Search::reset() for the following use cases:
- bench: needed to guarantee deterministic bench (ie. if you call bench from
interactive command line twice in a row you get the same value).
- Clear Hash: restore clean search state, which is the purpose of this button.
- ucinewgame: ditto.

No functional change.

Resolves #346

src/search.h

@@ -106,6 +106,7 @@ extern StateStackPtr SetupStates;
 
 void init();
 void think();
+void reset();
 template<bool Root> uint64_t perft(Position& pos, Depth depth);
 
 } // namespace Search