Improve connection handling

1. Previously unhandled connections will stay in pending state. It won't
be closed until timeout happened. This may lead to wasting system
resources. Now the (over-limit) connection is actively rejected.

2. When out-of-memory occurs here, reject the new connection instead of
throwing exception and crash.

3. Also clean up some unused bits.

PR #20961.

src/base/http/connection.h

@@ -47,10 +47,11 @@ namespace Http
 
     public:
         Connection(QTcpSocket *socket, IRequestHandler *requestHandler, QObject *parent = nullptr);
-        ~Connection();
 
         bool hasExpired(qint64 timeout) const;
-        bool isClosed() const;
+
+    signals:
+        void closed();
 
     private:
         static bool acceptsGzipEncoding(QString codings);