Various small fixes for speedup and consistency -- requires real review

* removes unneeded defaults for lists
* attempts to optimize the common case where lists on refreshed objects are unchanged (and thus don't need `replace`-ing
* replaces `Model#init` code using `removeAttr` to remove `null`  attributes with code in `Cacheable.model`
  - this should be faster and more consistent because:
      * attributes are not set before being removed
      * previously, code in `init` was not being run on all attributes when object stubs were loaded first, which means unsightly `null` values still appear in the UI (for, e.g., Documents in the selector)