[BDG-FORMATS-29] Re-organize the Feature schema

This is attempting to re-organize the Feature schema, along the
discussions that we (Timothy, Uri, Matt, Frank) have had in email and on
the phone.  The main requirements are:
- less file-format dependence in the field choice ('qValue'-like fields
  could be relegated to the 'attributes' field)
- fewer fields to improve the memory footprint
Is it ever possible that an object can have one accession in multiple databases?  I don't know if we're talking about a 7-sigma issue here, but is it worth going back to your original proposal of having an array of Dbxref objects?