Optimising lookup function
This MR adds two optimisations:
- lookup now uses KIO::stat instead of KIO::listDir.
- Switches to a larger (14Mib) buffer for the TransferJob. The docs say that 1Mib should be the maximum, but newer discussion has pointed to the fact that it can be enlarged to at least 14Mib: https://phabricator.kde.org/D15448