CLI sandbox and command clean up

A few changes:

 * Console commands use the canonical name for accessing the DM helper (see: #637). Folks can still use the "dm" alias in their own commands, but it makes sense for the core ODM classes to not rely on that.
 * Sandbox documents now use ArrayCollection for empty Reference/EmbedMany fields, which is an advisable best practice.
 * `AnnotationDriver::create()` only constructs a Reader instance if the argument is strictly null (small BC change here, but it shouldn't be an issue for anyone).