Fix "QIODevice::read" warnings.

Summary:
Calling readAll() after close() leads to this warning because QIODevice checks
the open status before reading, and warns (and returns -1) if closed.

Let's just create a different socket in doReconnect to make sure we have
no interference from the just closed one.

Test Plan: `ctest --verbose -R sqlite-tagtest` no longer has all these warnings.

Reviewers: dvratil, sengels

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D18558