xblock fields persist w/o breaking by scope

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.

@cpennington please review
@cahrens @sarina @dianakhuang can one of you please review?