Fix memory corruption in ScanFolder plugin

Summary:
Reasoning behind this patch is kinda dubious because I don't fully understand what happens, but it shouldn't make the matters worse either

Seems like deletion of KDirWatch objects from scan_folder in ~ScanThread(), after said QThread has been stopped, causes memory corruption.

Preemptively deleting scan_folder contents before we stop the thread resolves this.

also mark booleans shared across main thread and KDirWatch callbacks as atomic

Test Plan:
Without this patch unloading ScanFolder plugin sometimes crashes ktorrent, including unloading on exit
Doesn't happen anymore with this patch

Reviewers: stikonas

Reviewed By: stikonas

Differential Revision: https://phabricator.kde.org/D15265