- rewrote fetch_by_stable_id*() methods to look up current versions of stable
  IDs in *_stable_id tables
- new method lookup_current() to assist the former (also replaces
  _lookup_version())
- new method add_all_current_to_history() to implement this behaviour for
  StableIdHistoryTree

- new methods to get properties of current db:
  - get_current_release()
  - get_current_assembly()

- fixed _resolve_type() to copy with non Ensembl-style stable IDs (e.g.
  Anopheles)

- removed deprecated methods