QtNetwork: use QStringRef to optimize memory allocation

Replace substring functions that return QString with
corresponding functions that return QStringRef where
it's possible.

Change-Id: Id7f70e5bd72c91ed210ec463aad1d84fe3b40c15
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>

src/network/access/qftp.cpp

@@ -1218,7 +1218,7 @@ bool QFtpPI::startNextCmd()
 
     pendingCommands.pop_front();
 #if defined(QFTPPI_DEBUG)
-    qDebug("QFtpPI send: %s", currentCmd.left(currentCmd.length()-2).toLatin1().constData());
+    qDebug("QFtpPI send: %s", currentCmd.leftRef(currentCmd.length() - 2).toLatin1().constData());
 #endif
     state = Waiting;
     commandSocket.write(currentCmd.toUtf8());

src/network/access/qnetworkcookiejar.cpp

@@ -160,7 +160,7 @@ static inline bool isParentDomain(const QString &domain, const QString &referenc
     if (!reference.startsWith(QLatin1Char('.')))
         return domain == reference;
 
-    return domain.endsWith(reference) || domain == reference.mid(1);
+    return domain.endsWith(reference) || domain == reference.midRef(1);
 }
 
 /*!

src/network/kernel/qhostaddress.cpp

@@ -1015,7 +1015,7 @@ QPair<QHostAddress, int> QHostAddress::parseSubnet(const QString &subnet)
             netmask = parser.prefixLength();
         } else {
             bool ok;
-            netmask = subnet.mid(slash + 1).toUInt(&ok);
+            netmask = subnet.midRef(slash + 1).toUInt(&ok);
             if (!ok)
                 return invalid;     // failed to parse the subnet
         }

src/network/kernel/qnetworkproxy_win.cpp

@@ -323,7 +323,7 @@ static QList<QNetworkProxy> parseServerList(const QNetworkProxyQuery &query, con
         pos = entry.indexOf(QLatin1Char(':'), server);
         if (pos != -1) {
             bool ok;
-            uint value = entry.mid(pos + 1).toUInt(&ok);
+            uint value = entry.midRef(pos + 1).toUInt(&ok);
             if (!ok || value > 65535)
                 continue;       // invalid port number