Implement automated garbage collection for actor images

[joestubbs]

Currently, images are not removed at any time by Abaco processes, including on actor delete. One challenge is that multiple actors can be registered with the same image. Another issue is that images can be cached on any number of compute nodes, depending on which compute node the actor associated with the image has had workers running. Depending on the implementation we chose, there can also be race conditions if an actor (the last actor) referencing an image is deleted and then quickly re-registered with the same image. (This is a more common user pattern than one might expect).

Nevertheless, the lack of image management is becoming an issue in production; with increased usage, disk space on the compute nodes is filling up.