Core/Mysql: Multiple changes

- added support for setting NULL column values to MySQL wrapper in core
- replaced nonstandard strdup function and manual memory management with std::string
- fixed bug in MySQLPreparedStatement::getQueryString method, it failed when string parameter contained embedded '?' symbols
- fixed memory leak in MySQLPreparedStatement::setString method

src/server/shared/Database/MySQLConnection.h

@@ -62,9 +62,7 @@ struct MySQLConnectionInfo
     std::string port_or_socket;
 };
 
-typedef std::map<uint32 /*index*/, std::pair<const char* /*query*/, ConnectionFlags /*sync/async*/> > PreparedStatementMap;
-
-#define PREPARE_STATEMENT(a, b, c) m_queries[a] = std::make_pair(strdup(b), c);
+typedef std::map<uint32 /*index*/, std::pair<std::string /*query*/, ConnectionFlags /*sync/async*/> > PreparedStatementMap;
 
 class MySQLConnection
 {