feat(interface) ThreadSafeDatastoreCloser

Expose `Close()` to make it possible to clean up LevelDB instances.

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>
I want to keep the `Closer` interface outside of the `Datastore*` interfaces. I think the leveldb instance (or whatever you want the `Close` func in) can just conform to `io.Closer`.