Add --delete-after for sync command

Allow user to specify --delete-after option, used by sync command to delay deleting removed files until after new files have been uploaded. This is necessary for proper operation of a yum repository (and many other models) during the interim period when a sync has started but not all the metadata has yet been synced out.  In this case, the still-valid repodata/ directory files get deleted before the new content (packages and new repodata/ files) get uploaded, causing the mirror to be useless in the meantime, which could be many hours.