Async artifact download

Make artifact download asynchronous.  This necessitates splitting the api operation create into create + start.  The harpoonctl agent create command now only performs create and then waits for that operation to complete.  There is now a harpoonctl agent run command that performs create followed by run if the create is successful.
Hmm, I'm not a huge fan of giving each container a full reference to its "parent" registry. Is there a way to decouple this a little more? Maybe something like: defining a Remover interface, which the registry can satisfy, and passing it as a parameter to Create, rather than the container constructor?