XWIKI-9710: While filesytem is on, attachments are deleted, but folders in storage are not
XWIKI-10431: In major installations, Filesystem Attachment Store deleteEmptyDirs() can block startup for significant time.
Allow disabling of deleteEmptyDirs()
XWIKI-10431: In major installations, Filesystem Attachment Store deleteEmptyDirs() can block startup for significant time
deleteEmptyDirs() should always run in it's own thread.