Improve P2P discovery (openethereum#9526)

* Add `target` to Rust traces

* network-devp2p: Don't remove discovery peer in main sync

* network-p2p: Refresh discovery more often

* Update Peer discovery protocol

* Run discovery more often when not enough nodes connected

* Start the first discovery early

* Update fast discovery rate

* Fix tests

* Fix `ping` tests

* Fixing remote Node address ; adding PingPong round

* Fix tests: update new +1 PingPong round

* Increase slow Discovery rate
Check in flight FindNode before pings

* Add `deprecated` to deprecated_echo_hash

* Refactor `discovery_round` branching

Author: ngotchac

ethcore/sync/src/chain/propagator.rs

@@ -200,7 +200,7 @@ impl SyncPropagator {
 								let appended = packet.append_raw_checked(&transaction.drain(), 1, MAX_TRANSACTION_PACKET_SIZE);
 								if !appended {
 									// Maximal packet size reached just proceed with sending
-									debug!("Transaction packet size limit reached. Sending incomplete set of {}/{} transactions.", pushed, to_send.len());
+									debug!(target: "sync", "Transaction packet size limit reached. Sending incomplete set of {}/{} transactions.", pushed, to_send.len());
 									to_send = to_send.into_iter().take(pushed).collect();
 									break;
 								}