implemented the first version of statement_cache

include/matador/sql/abstract_sql_logger.hpp

@@ -1,21 +1,22 @@
 #ifndef MATADOR_BASIC_SQL_LOGGER_HPP
 #define MATADOR_BASIC_SQL_LOGGER_HPP
 
+#include <memory>
 #include <string>
 
 namespace matador::sql {
 
 /**
- * @brief Base class for sql logging
+ * @brief Base class for SQL logging
  *
  * This class acts as a base class to
- * implement a concrete logger for sql
+ * implement a concrete logger for SQL
  * statements.
  *
  * It provides interfaces to handle
  * the establishing and closing of
  * a database connection as well as
- * when a sql statement is about to
+ * when a SQL statement is about to
  * execute or going to be prepared.
  */
 class abstract_sql_logger
@@ -94,6 +95,8 @@ public:
   void on_prepare(const std::string &) override { }
 };
 
+const auto null_logger = std::make_shared<null_sql_logger>();
+
 }
 
 #endif //MATADOR_BASIC_SQL_LOGGER_HPP