Use the player's event loop for their server connections.

This approach reduces context switching, giving us that much extra boost
in throughput.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -41,6 +41,7 @@ import com.velocitypowered.proxy.util.VelocityChannelRegistrar;
 import com.velocitypowered.proxy.util.ratelimit.Ratelimiter;
 import com.velocitypowered.proxy.util.ratelimit.Ratelimiters;
 import io.netty.bootstrap.Bootstrap;
+import io.netty.channel.EventLoopGroup;
 import java.io.IOException;
 import java.net.InetSocketAddress;
 import java.nio.file.Files;
@@ -239,6 +240,10 @@ public class VelocityServer implements ProxyServer {
     return this.cm.createWorker();
   }
 
+  public Bootstrap initializeGenericBootstrap(EventLoopGroup group) {
+    return this.cm.createWorker(group);
+  }
+
   public boolean isShutdown() {
     return shutdown;
   }

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

@@ -71,7 +71,9 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
    */
   public CompletableFuture<ConnectionRequestBuilder.Result> connect() {
     CompletableFuture<ConnectionRequestBuilder.Result> result = new CompletableFuture<>();
-    server.initializeGenericBootstrap()
+    // Note: we use the event loop for the connection the player is on. This reduces context
+    // switches.
+    server.initializeGenericBootstrap(proxyPlayer.getMinecraftConnection().eventLoop())
         .handler(new ChannelInitializer<Channel>() {
           @Override
           protected void initChannel(Channel ch) throws Exception {

proxy/src/main/java/com/velocitypowered/proxy/network/ConnectionManager.java

@@ -108,15 +108,21 @@ public final class ConnectionManager {
         });
   }
 
+  public Bootstrap createWorker() {
+    return this.createWorker(this.workerGroup);
+  }
+
   /**
    * Creates a TCP {@link Bootstrap} using Velocity's event loops.
    *
+   * @param group the event loop group to use
+   *
    * @return a new {@link Bootstrap}
    */
-  public Bootstrap createWorker() {
+  public Bootstrap createWorker(EventLoopGroup group) {
     return new Bootstrap()
         .channel(this.transportType.socketChannelClass)
-        .group(this.workerGroup)
+        .group(group)
         .option(ChannelOption.TCP_NODELAY, true)
         .option(ChannelOption.CONNECT_TIMEOUT_MILLIS,
             this.server.getConfiguration().getConnectTimeout())