Imitated fetching the document from the database.
        Doesnt implement options from paisley to get the old revision or
        get the list of revision.

def open_doc(self, doc_id):
        '''Imitated fetching the document from the database.
        Doesnt implement options from paisley to get the old revision or
        get the list of revision.
        '''
        d = defer.Deferred()
        self.increase_stat('open_doc')
        try:
            doc = self._get_doc(doc_id)
            doc = copy.deepcopy(doc)
            if doc.get('_deleted', None):
                raise NotFoundError('%s deleted' % doc_id)
            d.callback(Response(doc))
        except NotFoundError as e:
            d.errback(e)

        return d