validator-discovery: basic retrying logic (#3059)

* validator_discovery: less flexible, but simpler design

* fix test

* remove unused struct

* smol optimization

* validator_discovery: basic retrying logic

* add a test

* add more tests

* update the guide

* more test logic

* Require at least 2/3 connectivity.

* Fix test.

* Update node/network/gossip-support/src/lib.rs

Co-authored-by: André Silva <[email protected]>

* Update node/network/gossip-support/src/lib.rs

Co-authored-by: André Silva <[email protected]>

Co-authored-by: Robert Klotzner <[email protected]>
Co-authored-by: Robert Klotzner <[email protected]>
Co-authored-by: André Silva <[email protected]>