[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
Last time, you mentioned the possibility of multiple accessions in a single database.  Does this reflect your resolution of this potential issue?