Fix for multi agent job completion

[msm-code]

Your checklist for this pull request

• [x] I've read the contributing guideline.
• [x] 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? With multiple agents, jobs sometimes don't complete properly.

What is the new behaviour? Jobs complete properly. I hope, at least.

This is a draft, because I need to set-up a proper mquery testing lab (again). Daemon is notoriously hard to reason about, and I really should test this code before the release.

This is related to PR #271, but I think this is a more correct solution - I'll explain "why" when I test this properly.

Test plan Create a setup with multiple agent groups and ursadb, and run a single query there. It should complete properly in all cases (indepentent of how many samples are in each agent group)

Closing issues

fixes #270