Fix MQTT re-subscription logic

[OttoWinter]

Description:

The old re-subscription logic was wrong and re-subscribing when the topic didn't change (see log in attached issue). I didn't step through the old code to figure out what the issue was because it was quite messy and hard to understand. But I suspect it's two things combined:

• the comparison of the message callback (will never be the same if an anonymous method)
• the unsub() callback directly followed by the re-subscribe coroutine (a race condition)

Instead, I rewrote the code to be (I think) easier to understand using some nice attr data classes.
Using some quick testing I was able to confirm that the issue is resolved with this.

Also, the tests seem to be incorrect. I manually stepped through them and couldn't understand why the previous checks were there. The new comparisons look more correct to me when manually stepping through. Nevermind (the tests are way too long and should probably be split up at some point)

(if there's another patch release, it would be great to include it there)

Related issue (if applicable): fixes #18909

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

CC: @emontnemery @balloob

[balloob]

You can merge it when you're ready.