Use reachdown instead of:

Two notable differences:

• reachdown does not do if (typeof db.down === 'function') return db.down(type). That's OK, we didn't add a down() method anywhere nor did we advertise this ability.
• subleveldown now doesn't just get the innermost db (db.db.db..). Instead, it traverses down until db is not a levelup, deferred-leveldown or encoding-down instance. This means, if there are additional layers (e.g. encrypt-down, cachedown) subleveldown stops there, not necessarily at the "storage layer".

Canary-tested against cabal-core (which uses the old memdb, I'll be sending them a PR to switch to level-mem).

This PR fails on node 6 because of a new test on encrypt-down which uses ES6. I'll see if I can make that test dependency-free by mocking an abstract-leveldown.