Switch to being a Discovery provider instead of using add_node_addr

[flub]

Currently gossip uses add_node_addr to add addressing information from other nodes that it learned via gossiping(?). There is an observed issue with this that it may gossip a different Relay URL compared to the Relay URL a node may already use for the same remote node. That results in some unexpected warnings but otherwise does not do much harm.

However, add_node_addr is also soft-deprecated by now because it directly modifies the internal state of the magic socket.

It might be much nicer if gossip would also be a discovery provider itself, and expose the addressing information it has about other nodes this way to the magic socket.

[b5]

I believe #108 closes this