Fix a counter race condition

[msm-code]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running mquery, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?
Tasks are added first, and agent job counter is incremented second. This makes it possible for the counter to decrease below zero, cancelling the task.

What is the new behaviour?
Fixed the order and removed the race.

Test plan

Closing issues