Replica should have a way to say it is behind master

The only way I know of to view slave status is with the command show slave status;
That means three important values that could be used by scripts or by maxscale are not available:

• Slave_SQL_Running_State
• Seconds_Behind_Master
• Gtid_IO_Pos

There is no way to SHOW SLAVE STATUS INTO @MYVAR;

These values should be made available in global status.

Is there a way to see if the slave is still applying relay logs (behind master)? A global status variable would be a possible method. This could be used by maxscale to know not to promote a slave to master because it has not yet finished applying relay logs.

Related Jira issue: MXS-4798