Ref Level/community#101

In other states (besides 'open') a 'Database is not open' error is
now thrown. This reduces the scope of `deferred-leveldown` to what's
strictly needed in `levelup` and aligns behavior.

In addition, override public methods of `abstract-leveldown` instead
of private methods. This has one downside: they need to do the same
callback to promise conversion that `abstract-leveldown` does. The
upside is that operation callbacks are not called before the db has
finished opening, including in cases where `abstract-leveldown` has
a fast-path, like on `db.batch([])` which because the array is
empty bypasses `_batch()`.

Closes #91
Closes #90