fix: connect to backend immediately on channel active for server-first protocols(#794) (#813)

- Closes #794 
- TCP port forwarding currently fails for server-first protocols (SMTP,
  FTP, SSH, PostgreSQL) because the backend connection is only established
  when the client sends data.

Sources/SocketForwarder/ConnectHandler.swift

@@ -35,9 +35,6 @@ extension ConnectHandler: ChannelInboundHandler {
     typealias OutboundOut = ByteBuffer
 
     func channelRead(context: ChannelHandlerContext, data: NIOAny) {
-        if self.pendingBytes.isEmpty {
-            self.connectToServer(context: context)
-        }
         self.pendingBytes.append(data)
     }
 
@@ -46,6 +43,12 @@ extension ConnectHandler: ChannelInboundHandler {
         self.log?[metadataKey: "proxy"] = "\(context.channel.localAddress?.description ?? "none")"
         self.log?[metadataKey: "server"] = "\(context.channel.remoteAddress?.description ?? "none")"
     }
+
+    func channelActive(context: ChannelHandlerContext) {
+        self.log?.trace("frontend - channel active, connecting to backend")
+        self.connectToServer(context: context)
+        context.fireChannelActive()
+    }
 }
 
 extension ConnectHandler: RemovableChannelHandler {