MXS-852: Use stored query type for COM_STMT_EXECUTE

When a COM_STMT_EXECUTE or a COM_STMT_SEND_LONG_DATA command is executed,
the query type of the prepared statement is used. This allows read-only
prepared statements to be load balanced across slaves.

server/modules/protocol/MySQL/mysql_common.c

@@ -1637,7 +1637,7 @@ bool mxs_mysql_extract_ps_response(GWBUF* buffer, MXS_PS_RESPONSE* out)
     return rval;
 }
 
-uint32_t mxs_mysql_extract_execute(GWBUF* buffer)
+uint32_t mxs_mysql_extract_ps_id(GWBUF* buffer)
 {
     uint32_t rval = 0;
     uint8_t id[MYSQL_PS_ID_SIZE];