Refactoring: changes ModSecurityStringVar to transaction::Variable

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

src/variables/tx.h

@@ -34,7 +34,7 @@ class Tx : public Variable {
     explicit Tx(std::string _name)
         : Variable(_name) { }
 
-    std::list<ModSecurityStringVar *> *
+    std::list<transaction::Variable *> *
         evaluate(Assay *assay) override;
 };