Port away from KTcpSocket to QSslSocket

Test Plan:
Tested TLS and plain connection against my Postfix server and SSL connection
against Gmail.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: vkrause, kde-pim

Tags: KDE PIM

Maniphest Tasks: T11533

Differential Revision: https://phabricator.kde.org/D23925