Expose & visualise distribution buffer busy limit - zdbbl

[gerhard]

This will be closed after TGIR S01E04 gets recorded. The goal is to demonstrate how to do this, and then let an external contributor have a go.

Before this patch, the Data buffered in the distribution links queue graph was empty.

This is what that graph looks like after this gets applied:

References

• RabbitMQ Runtime Tuning - Inter-node Communication Buffer Size
• erl +zdbbl

Fixes #39