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/uci.cpp

@@ -27,7 +27,6 @@
 #include "search.h"
 #include "thread.h"
 #include "timeman.h"
-#include "tt.h"
 #include "uci.h"
 
 using namespace std;
@@ -181,7 +180,7 @@ void UCI::loop(int argc, char* argv[]) {
 
       else if (token == "ucinewgame")
       {
-          TT.clear();
+          Search::reset();
           Time.availableNodes = 0;
       }
       else if (token == "isready")    sync_cout << "readyok" << sync_endl;