First nuke_dir() creates a folder in the .delete directory and moves all files
in the delete target into this folder. This is because the files may still be open
and windows only allows moving open files and not the parent folder. Then move the
directory into the delete dir and finally call nuke_dir_sync() in the background.
We sleep 5 seconds before actually deleting the folder to give the view group indexer
a chance to close its files.
On startup use nuke_dir_sync to clean the .delete directory because it now can contain