Alright, after discussion, I'm approving this as-is due to the size of the PR and complexity of the code, with the understanding that the remaining comments must be addressed in follow up PRs. Some of my comments are of minor impact and open for discussion (e.g.: use of smart ptrs is meant to reduce code fragility), some of my comments are of high impact and eventual must-haves (e.g.: about the define that disables the extern statement for the global MDNS instance).
A general issue should be opened to track the follow up work as a whole, but we can discuss individual comments here for now.
Let's get this one merged!