Faster simplejob start

Summary: Changing the signals connections to the new syntax. From being expensive to not being visible in kcachegrind.

Test Plan: kioclient5 move of 3000 files and move of 3000 files in dolphin to different disks (no rename).

Reviewers: Frameworks, mwolff, dfaure

Reviewed By: dfaure

Subscribers: mwolff, broulik, ngraham, anthonyfieroni

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10124