For download-then-kill strategy, this waits for all changing images on the target
release to be available on device before killing the old services. This
will prevent that multicontainer applications get to a state where some
services of the new release start runnning much before others have been
downloaded.
When adding new services to a multicontainer app, the supervisor will
now wait for other changing services to be downloaded before starting
the new service.
Closes: #1812
Change-type: patch
7aedc97ee1