Drop support of memdb

[vweevers]

In favor of the newer level-mem. I already planned to drop memdb; I'm writing this because @bcomnes asked for some context in bcomnes/level-hookdown#29 (comment).

Warning: this is a long story. You can skip it if you accept "memdb is old" as a good enough reason 😉

When we moved subleveldown to the Level org and updated it to work with latest Level modules, we did not account for memdb (as level-mem was already the preferred module). It's pure luck that wrapping memdb with subleveldown still works today. I recently added an integration test (#75) to avoid breaking it (after seeing that memdb is still depended upon). However, we're now at a point where continued support is just madness.

A typical db passed to subleveldown consist of several "layers". E.g. leveldown wrapped in encoding-down wrapped in deferred-leveldown wrapped in levelup. Behind the scenes, subleveldown unwraps that db to get to leveldown, then wraps it in a layer that prefixes keys, then (re)wraps that with encoding-down and levelup. For more background, see Level/community#82.

The problem with memdb is that it depends on an old version of deferred-leveldown (< 2.0.0) which when closed, doesn't expose its underlying db. So subleveldown can only unwrap after opening.

This limitation means we can't add a manifest (Level/community#83), which allows to inspect what features a db supports via db.supports. For example, db.supports.permanence indicates whether the underlying storage is persistent. Would be true for leveldown, false for memdown. For subleveldown to determine that, it must inspect the manifest of the unwrapped db, and this must happen synchronously, possibly before the db is open. Then you would be able to do:

var sub = require('subleveldown')(db)

if (!sub.supports.permanence) {
  throw new Error('Persistent storage is required')
}

If we drop support of memdb, we can unwrap synchronously and expose a manifest.

Manifests also make the long-term merging of levelup with abstract-leveldown (Level/community#58) easier. They allow us to determine which features a db already has and skip adding those. Because at some point, db's will be a mix of level(up) and abstract-leveldown features. Eventually, the unwrapping in subleveldown will also become simpler (thanks to needing less layers). Full circle :)