Fix session resumption with OpenSSL 1.1

OpenSSL 1.1.0 and higher requires SSL_shutdown to mark a
session as resumable.

QHttpNetworkConnection/Channel tries to re-use one shared SSL
context (and the session) for its 'channels'. The session is
marked as non-resumable without shutdown sent/received.
This makes it useless for QHttpNetworkConnection.

See: https://github.com/openssl/openssl/issues/1550
Fixes: QTBUG-71967
Change-Id: Iaaceb18c4c5a090f997f9850981a27f04f1f8b06
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/ssl/qsslsocket_openssl.cpp

@@ -418,6 +418,10 @@ bool QSslSocketBackendPrivate::initSslContext()
 void QSslSocketBackendPrivate::destroySslContext()
 {
     if (ssl) {
+        // We do not send a shutdown alert here. Just mark the session as
+        // resumable for qhttpnetworkconnection's "optimization", otherwise
+        // OpenSSL won't start a session resumption.
+        q_SSL_shutdown(ssl);
         q_SSL_free(ssl);
         ssl = nullptr;
     }