Breaking: parent db must support deferredOpen

[vweevers]

Parent database must support deferredOpen

By parent we mean:

var parent = require('level')('db')
var sublevel = require('subleveldown')(parent, 'a')

By deferredOpen we mean that the db opens itself and defers operations until it's open. Currently that's only supported by levelup (and levelup factories like level). Previously, subleveldown would also accept abstract-leveldown db's that were not wrapped in levelup.

Better isolation

Opening and closing a sublevel no longer opens or closes the parent db. The sublevel does wait for the parent to open (which in the case of levelup already happens automatically) but never initiates
a state change on the parent.

If one closes the parent but not the sublevel, subsequent operations on the sublevel (like get and put) will yield an error, to prevent segmentation faults from underlying stores.

Drops support of old modules

• memdb (use level-mem instead)
• deferred-leveldown < 2.0.0 (and thus levelup < 2.0.0)
• abstract-leveldown < 2.4.0

Rejects new sublevels on a closing or closed database

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

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

db.close(function () {})

---

Closes #84, closes #83, closes #60 and paves the way for Level/community#83.

[vweevers]

FYI @bcomnes @christianbundy @noffle

[vweevers]

5.0.0