If a container doesn't exist in the memdb, First will return nil, not an
error. This should be checked for before using the result.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
Reuse existing structures and rely on json serialization to deep copy
Container objects.
Also consolidate all "save" operations on container.CheckpointTo, which
now both saves a serialized json to disk, and replicates state to the
ACID in-memory store.
Signed-off-by: Fabio Kung <fabio.kung@gmail.com>
