Refactoring on the operators: negation is now being handled globally

Other minors changes were also made, including adding the prefix `m_'
to all the members of the class.

src/operators/verify_ssn.cc

@@ -32,8 +32,8 @@ bool VerifySSN::evaluate(Transaction *transaction, const std::string &str) {
 
 VerifySSN::VerifySSN(std::string op, std::string param, bool negation)
     : Operator() {
-    this->op = op;
-    this->param = param;
+    this->m_op = op;
+    this->m_param = param;
 }
 
 }  // namespace operators