Fix nondiscard warnings with lock_guard

[chapulina]

🦟 Bug fix

Summary

Clean up Windows warnings.

I changed to keeping the object instead of discarding it, let's see if that's enough. This pattern is used on other plugins already.

The original line:

std::lock_guard<std::mutex>(this->dataPtr->mutex)

Was creating an object but not holding it. This would be a way to keep it within the function's scope:

auto lock = std::lock_guard<std::mutex>(this->dataPtr->mutex)

I think some compilers may call a constructor, and then a copy operator that way. So I chose to go with the following, which only calls the constructor:

std::lock_guard<std::mutex> lock(this->dataPtr->mutex)

Let me know what you think, @jennuine .

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
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• All tests passed (See test coverage)
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[jennuine]

LGTM, is it possible to run on win-beefy to see if changes worked?

[chapulina]

Retriggered the Windows build. I think it should run on beefy this time, because that's currently the only node with the win label, see "Label win matches 1 node." here:

https://build.osrfoundation.org/job/ign_gui-pr-win/configure

CC @j-rivero