Destroying the store now:

- unload's all Records
- unload's all RecordArrays
- destroy's all records (so life cycle hooks get called for cleanup)
- destroy's all RecordArrays

Unloading a record now:
- is allowed, unless the record is inflight. (previously it was disallowed for all dirty states)

also, now once a record has been destroyed attempts to unload it will be
black holed
Merge pull request #1714 from stefanpenner/cleanup_when_store_destroys

Destroying the store cleanups up after itself: