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/benchmark.cpp

@@ -104,7 +104,7 @@ void benchmark(const Position& current, istream& is) {
 
   Options["Hash"]    = ttSize;
   Options["Threads"] = threads;
-  TT.clear();
+  Search::reset();
 
   if (limitType == "time")
       limits.movetime = stoi(limit); // movetime is in ms