Refactoring: changes ModSecurityStringVar to transaction::Variable

Having the variables and collection in place before start to implement
persistent storage.

src/variables/time_sec.h

@@ -33,7 +33,7 @@ class TimeSec : public Variable {
     explicit TimeSec(std::string _name)
         : Variable(_name) { }
 
-    std::list<ModSecurityStringVar *> *
+    std::list<transaction::Variable *> *
         evaluate(Assay *assay) override;
 };