Registry: Implement server-side seeding

[Geod24]

When a node first start, from a clean configuration, it needs to connect to a set of peer and download the chain.
We implemented this client side a while ago (#19) but it was never implemented server side.

We should be able to do an A / AAAA query on a zone (probably the realm itself, but open to discussion, as the same idea might apply to flash) and get a "well distributed" set of nodes. Well distributed meaning that at least X validators are included (X to be decided based on the number of active validators), and the rest being staking nodes (ideally, but this requires #2867). Obviously there must be a maximum numbers of nodes that will be included in the response, and the nodes should be round-robin-ed on a timely basis (not per request, I think, to avoid people using it as a way to discover the network).

Note that the main part of this issue can be done without #2867 : it's fine if the initial implementation just return validators.