KSieve: don't call connectToHost twice for each host on startup.

Summary:
The code to reconnect after a disconnect shouldn't just assume
that "m_connected == false" means we're disconnected. There's a 3rd case:
we are trying to connect. So I introduced another boolean, m_disconnected.

When disconnected, m_connected == false, m_disconnected == true.
While trying to connect, m_connected == false, m_disconnected == false.
When connected, m_connected == true, m_disconnected == false.

Test Plan: Watching kmail's debug output

Reviewers: mlaurent

Subscribers: #kde_pim

Differential Revision: https://phabricator.kde.org/D2172