Fix NonUniqueResultException thrown by JobManager#findJobForRelatedEntity if multiple results are found

[ocrampete16]

A Doctrine\ORM\NonUniqueResultException is thrown by JobManager#findJobForRelatedEntity if there are multiple instances of the same command that share the same related entity.

Looking at some of the other methods in this class that explicitly restrict the SQL result set to 1 row if only one entity is expected, I thought adding a limit to this method made sense.