Fix checkpoint exchange to support rolling upgrades, fixed silent lag startup period, Mixed v4.12/v4.13 peers coexist

src/p2p_server.cpp

@@ -3188,6 +3188,13 @@ bool P2PServer::P2PClient::on_checkpoint_request()
 	LOGINFO(4, "peer " << log::Gray() << static_cast<char*>(m_addrString) << log::NoColor()
 		<< " requested checkpoints, sending " << checkpoints.size() << " checkpoints");
 
+	// If we have no checkpoints, don't send a response (backward compatibility)
+	// Older peers expect count >= 1, so sending count=0 would cause them to disconnect
+	if (checkpoints.empty()) {
+		LOGINFO(4, "No checkpoints to send to peer " << static_cast<char*>(m_addrString) << ", skipping response");
+		return true;
+	}
+
 	// Send checkpoint response
 	const bool result = server->send(this,
 		[&checkpoints, this](uint8_t* buf, size_t buf_size) -> size_t

src/p2pool.cpp

@@ -1464,10 +1464,6 @@ void p2pool::download_block_headers4(uint64_t start_height, uint64_t current_hei
 					// Pre-fetch recent mainchain blocks to populate cache before sidechain sync
 					prefetch_mainchain_blocks(current_height);
 
-					// Retry verification of any unverified sidechain blocks now that mainchain data is available
-					// This catches up the sidechain immediately instead of waiting for the next ZMQ notification
-					m_sideChain->retry_unverified_blocks();
-
 					api_update_network_stats();
 					get_miner_data();