The current implementation of getRefs is a bit fragile. It returns a
slice to callers without copying its contents, and assumes the contents
will not be modified elsewhere.
Also, the current implementation of Dereference requires copying the
slice of references, excluding the one we wish to remove.
To improve both of these things, change refs to be a map of maps.
Deleting an item becomes trivial, and returning a slice of references
necessitates copying from the map.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
a762222396