Right now ping timeout is equal to operation timeout. We can reduce timeout for ping, because it does not perform any action on target node. This will allow to detect node disconnect faster. This timeout parameter should be added to config (check_timeout).

Additionally, it would be good to perform ping in parallel with FuturesUnordered. Right now it iterates nodes one-by-one: https://github.com/qoollo/bob/blob/master/bob/src/link_manager.rs#L61