Import Khepri-related preparation work

[dumbbell]

@dcorbacho and I have several changes in our respective branches that could go into master and then a RabbitMQ release way before we start to import Khepri bits. This would make the review of Khepri much much easier.

This would be mostly a reorganization of the code, no functional changes.

Consider this a "meta-issue" to track all changes we would like to import into master.