Allow a marker file on the target storage to suspend all running transfers.
This would be particularly helpful in situations where the target location is known to be not available for a certain period (like rebooting the file server(s) or similar).
It also needs to be taken into account that when e.g. the file server(s) hosting the target location are rebooting the file won't be visible to the service, so it should remember that state ("suspended") and only leave it once it can
- reach the target location
- the marker file is NOT present any more
NOTE: please also see the related issues for more things to consider on this issue!