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.cpp

@@ -181,6 +181,18 @@ void Search::init() {
 }
 
 
+/// Search::reset() clears all search memory, to restore a deterministic state
+
+void Search::reset () {
+
+    TT.clear();
+    History.clear();
+    CounterMovesHistory.clear();
+    Gains.clear();
+    Countermoves.clear();
+}
+
+
 /// Search::perft() is our utility to verify move generation. All the leaf nodes
 /// up to the given depth are generated and counted and the sum returned.
 template<bool Root>