Make embedded-relation-adapter the default

No Issue
- Generalize embedded-relation-adapter so it can be used for both fetching
  and saving records.
- Change Adapter inheritance hierarchy to
  RESTAdapter => BaseAdapter => EmbeddedRelationAdapter. ApplicationAdapter
  now extends EmbeddedRelationAdapter.
- Cleanup or remove adapters that existed just to extend directly
  from EmbeddedRelationAdapter.