Allow pt-slave-delay to start even when Slave SQL thread is not running

Sometimes it's mandatory to have a delayed slave and it's not allowed to reduce that delay or even worse catch up completely the lag between slave and master (#37570).

However if there is not much events in the binary log, the time elapsed between START SLAVE and starting pt-slave-delay can cause the replication to recover completely.

In an ideal world, it would be great to be able to start pt-slave-delay even when Slave_SQL_Running is OFF.

This will be useful when for example replication credentials need to be modified. SLAVE could be stopped, then change master to the new credentials and finally start only the IO thread.