Don't make requests if we didn't manage to connect

Summary:
Use QtConcurrent::run instead of QThread. QThread will issue the thread
immediately and that will overload the operating system giving "too many files
opened errors". QtConcurrent will do the appropriate balancing for us.

Test Plan: Doesn't crash on me anymore

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7785