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/ucioption.cpp

@@ -22,6 +22,7 @@
 #include <ostream>
 
 #include "misc.h"
+#include "search.h"
 #include "thread.h"
 #include "tt.h"
 #include "uci.h"
@@ -34,7 +35,7 @@ UCI::OptionsMap Options; // Global object
 namespace UCI {
 
 /// 'On change' actions, triggered by an option's value change
-void on_clear_hash(const Option&) { TT.clear(); }
+void on_clear_hash(const Option&) { Search::reset(); }
 void on_hash_size(const Option& o) { TT.resize(o); }
 void on_logger(const Option& o) { start_logger(o); }
 void on_threads(const Option&) { Threads.read_uci_options(); }