Breaking: remove Batch._levelup property

In favor of Batch.db which matches the property on
abstract-leveldown's chained batch interface.

Unlikely to cause breakage because _levelup was a private and
undocumented property.

lib/batch.js

@@ -6,8 +6,7 @@ const getCallback = require('./common').getCallback
 const getOptions = require('./common').getOptions
 
 function Batch (levelup) {
-  // TODO (next major): remove this._levelup alias
-  this.db = this._levelup = levelup
+  this.db = levelup
   this.batch = levelup.db.batch()
   this.ops = []
   this.length = 0
@@ -53,7 +52,7 @@ Batch.prototype.clear = function () {
 }
 
 Batch.prototype.write = function (options, callback) {
-  const levelup = this._levelup
+  const levelup = this.db
   const ops = this.ops
 
   callback = getCallback(options, callback)