mqtt enhancements - protocol version 5 support, mqtt node tests

[southbite]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ knolleary
❌ Simon Bishop

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[coveralls]

Coverage decreased (-0.5%) to 76.25% when pulling fdc2f95 on southbite:dev into 8abc5b3 on node-red:dev.

[southbite]

closing rather - trying to figure out how package-lock found its way in, and how coverage could possibly have decreased - as I have added tests where there were none?

[knolleary]

1. rebase your branch on the latest code from dev so it doesn't drag in other changes as part of the merge
2. don't worry about the exact coverage figures - if there's a genuine problem we'll discuss that when we review