Improvements for edge-cases for a few tasks
Improvements for edge-cases for a few tasks

* Properly handle broken symlinks or files vanishing during upload
* Mark retail as installed only after the files have been processed
* Avoid conflicts over flags.lch by making ApplyEngineFlagsTask single-threaded

MjnMixael/knossos,MjnMixael/knossos,MjnMixael/knossos,MjnMixael/knossos,MjnMixael/knossos