The algorithm is like so:
* get all the leaves of the graph, these are all the service that don't have any dependency
* once a service is started we take the list of its parents (dependents)
* if all the dependencies of each of those dependents are started then we can start it as well
* if not then we continue to the next dependent
Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>
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