Batch handshake and server login/ping packets

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/VelocityServerConnection.java

@@ -150,11 +150,12 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
       handshake.setServerAddress(registeredServer.getServerInfo().getAddress().getHostString());
     }
     handshake.setPort(registeredServer.getServerInfo().getAddress().getPort());
-    mc.write(handshake);
+    mc.delayedWrite(handshake);
 
     mc.setProtocolVersion(protocolVersion);
     mc.setState(StateRegistry.LOGIN);
-    mc.write(new ServerLogin(proxyPlayer.getUsername()));
+    mc.delayedWrite(new ServerLogin(proxyPlayer.getUsername()));
+    mc.flush();
   }
 
   public @Nullable MinecraftConnection getConnection() {

proxy/src/main/java/com/velocitypowered/proxy/server/PingSessionHandler.java

@@ -34,10 +34,12 @@ public class PingSessionHandler implements MinecraftSessionHandler {
     handshake.setServerAddress(server.getServerInfo().getAddress().getHostString());
     handshake.setPort(server.getServerInfo().getAddress().getPort());
     handshake.setProtocolVersion(ProtocolVersion.MINIMUM_VERSION);
-    connection.write(handshake);
+    connection.delayedWrite(handshake);
 
     connection.setState(StateRegistry.STATUS);
-    connection.write(StatusRequest.INSTANCE);
+    connection.delayedWrite(StatusRequest.INSTANCE);
+
+    connection.flush();
   }
 
   @Override