Add limited flush consolidation, if the remote server supports it.

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -124,6 +124,13 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     }
   }
 
+  @Override
+  public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
+    if (sessionHandler != null) {
+      sessionHandler.readCompleted();
+    }
+  }
+
   @Override
   public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
     if (ctx.channel().isActive()) {

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftSessionHandler.java

@@ -70,6 +70,10 @@ public interface MinecraftSessionHandler {
 
   }
 
+  default void readCompleted() {
+
+  }
+
   default boolean handle(AvailableCommands commands) {
     return false;
   }

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

@@ -168,12 +168,17 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
     if (packet instanceof PluginMessage) {
       ((PluginMessage) packet).retain();
     }
-    playerConnection.write(packet);
+    playerConnection.delayedWrite(packet);
   }
 
   @Override
   public void handleUnknown(ByteBuf buf) {
-    playerConnection.write(buf.retain());
+    playerConnection.delayedWrite(buf.retain());
+  }
+
+  @Override
+  public void readCompleted() {
+    playerConnection.flush();
   }
 
   @Override