end(callback) {
    const tx = this._transaction.end()
    if (typeof callback === 'function') {
      // XXX: Since Transaction#end is now synchronous, this needs to
      // asynchronously call the callback like Transaction#end used to.
      // Change this to be synchronous in the next major version.
      setImmediate(callback, tx)
    }
  }
End the transaction.

@param  {Function} callback