Validate and receive tip block without full view of public keys/address of all nodes.

[wuzhengy]

Mina node has no need to have a full view of public key/address to valid the winner block producer. Does this mean a node has to contact a certain NODE to receive tip block? The network communication on gossip style complexity is N square, if no special delegated NODE to provide such tip, the seeking for tip will incur large data consumption. It seems that a node does not really know other nodes existence either. Thanks.