add CReserveScript to allow modular script keeping/returning

- use one CReserveScript per mining thread

src/script/script.h

@@ -609,4 +609,13 @@ public:
     }
 };
 
+class CReserveScript
+{
+public:
+    CScript reserveScript;
+    virtual void KeepScript() {}
+    CReserveScript() {}
+    virtual ~CReserveScript() {}
+};
+
 #endif // BITCOIN_SCRIPT_SCRIPT_H