Letting xblocks handle scope rather than separating fields into
different attrs. Although, split still shunts content fields to a
different collection than setting and children fields.
The big difference is that content fields will always be a dict and not
sometimes just a string and there's no special casing of 'data' attr.
The other mind change is no more 'metadata' dict.
A new modulestore backed by mongo that changes the data format to
facilitate easy versioning, sharing content between courses, and fast
lookup of course structure and Scope.settings data.
Conflicts:
cms/djangoapps/contentstore/tests/test_contentstore.py
