Require deferredOpen support

[vweevers]

To simplify the open/close handling on sublevels (and fix issues like #60) I propose the following.

1. Only accept a db that supports deferredOpen (currently that's only levelup, in the future also abstract-leveldown). In subleveldown we then never open or close the parent db. More precisely: we never initiate a state change. The parent db must open itself (or once closed by the user, be explicitly reopened by the user) because sublevels shouldn't touch (the state of) the rest of the db.
2. We can then have subdown._open() wait for an open event of the parent db. It would be nice to not have an _open() method at all, i.e. sublevels not having to care about whether the db is open, because deferredOpen support means you can always do operations on it, but this currently can't work because we operate on an unwrapped db. When abstract-leveldown gets deferredOpen support, things will get better.

[vweevers]

The following cases will become illegal.

Creating a sublevel on a closed db

db.close(function (err) {
  subdb(db, 'example').on('error', function (err) {
    throw err // Error: Parent database is not open
  })
})

Creating a sublevel on a closing db / Closing a db while sublevel is opening

subdb(db, 'example').on('error', (err) => {
  throw err // Error: Parent database is not open
})

db.close(function () {})