[Dashing] Avoid reference cycle between Context and GraphListener

[ivanpauno]

Backport of #906.

This change breaks ABI of the GraphListener class.

That class is never used directly by an user.
In the internal places where that class is being used, it's being stored as a shared_ptr.
i.e.: ABI of other objects shouldn't be affected, and thus the user shouldn't be affected.

I'm not quite sure if we should backport a fix like this or not as it's hard to verify if it actually breaks "user" ABI or not.
If the answer is no, I will delete this from Dashing and Eloquent patch release board.

[wjwwood]

I would decline to do this, even with the fair points about the class not being used externally and therefore not a high risk ABI break.

Is someone asking for this fix or we just think it's good to have?

[ivanpauno]

Is someone asking for this fix or we just think it's good to have?

I think nobody is asking for it.
I've added it to the board in the past, and I wanted to double check what we should do in these cases.

I will close and delete it from the Dashing backports board.
If someone specifically asks for this in the future we can reconsider making the backport.

[wjwwood]

Ok, yeah perhaps someone on our team would disagree, but I'd prefer to document the issue rather than break ABI, in this case.

Perhaps here, if appropriate: https://index.ros.org/doc/ros2/Releases/Release-Dashing-Diademata/#known-issues

[ivanpauno]

Ok, yeah perhaps someone on our team would disagree, but I'd prefer to document the issue rather than break ABI, in this case.

Perhaps here, if appropriate: https://index.ros.org/doc/ros2/Releases/Release-Dashing-Diademata/#known-issues

Sounds like a good idea, I've opened ros2/ros2_documentation#826 to document the issue.