Use setImmediate with callback in _close()

[ralphtheninja]

Closes #109

[vweevers]

Breaks the destroy tests.

[vweevers]

Breaks the destroy tests.

I suspect that has something to with deferred-leveldown. When the db (a levelup instance) has closed, db.db points to an instance of deferred-leveldown instead of level-js.

https://github.com/Level/level.js/blob/fb185a978aaee2f5b0d5b106838a4ea1798873e1/test/custom-test.js#L125

[vweevers]

Confirmed. We have a few options:

1. Remove the ability to destroy(db). User must do destroy(location[, prefix]). I prefer this.
2. Require the user to do destroy(db._db) (where db is levelup, and db._db is level-js)
3. Require the user to keep a reference to the level-js db themselves so that they can do destroy(db)
4. Hack: find the level-js instance: if (!db.prefix && db._db) db = db._db (where db is either level-js, deferred-leveldown or levelup).

[ralphtheninja]

Yes I just found the same problem. Since we do setImmediate() then db.db is DeferredLevelDOWN object. I wouldn't mind picking option 1 either.

[vweevers]

I wouldn't mind picking option 1 either.

Let's go with that 👍

[ralphtheninja]

I'll keep this PR open and fix that first.