TCPSlaveBase: deprecate socket() and introduce tcpSocket()
It looks like the socket() method returns a QIODevice* is mostly due to historical reasons (from the kdelibs days); and it's also obvious that most sub-classes of TCPSlaveBase that use that method, cast the pointer returned by it to QSslSocket*, so that they can e.g. connect to the QAbstractSocket::connected() signal.
So deprecate socket() and add a tcpSocket() method that returns a QAbstractSocket*; the reason for not returning a QSslSocket (the type of d->socket is actually QSslSocket) is, as Volker explained to me on IRC, since socket() returns a QIODevice rather than a KTCPSocket, which meant that the latter stayed as an implementation detail, porting to QSslSocket was made easier, so we go one step down the hierarchy, and return a QAbstractSocket*.
Port HTTPProtocol to the new method.