api: support bulk updates

[slint]

Currently, the API supports bulk index and delete. It would be nice to have a bulk_update, which would allow partial updates to indexed documents. The API calls could look something like this:

class RecordIndexer(object):
...
    def bulk_update(self, record_patch_iterator):
        # record_patch_iterator is a iterable of 2-tuples with a record UUID 
        # and a dictionary with the patch/update (see example below)
        self._bulk_op(record_patch_iterator, 'update')

    def _bulk_op(self, record_id_iterator, op_type, index=None, doc_type=None):
        with self.create_producer() as producer:
            if op_type == 'update':
                for rec, patch in record_id_iterator:
                producer.publish(dict(
                    id=str(rec),
                    patch=patch,
                    op='update',
                    index=index,
                    doc_type=doc_type
                ))
            else:
                ...
    
    def _update_action(self, payload):
        ...
        return {
            '_op_type': 'delete',
            '_index': index,
            '_type': doc_type,
            '_id': payload['id'],
            'doc': payload['patch']
        }

RecordIndexer().bulk_update(
    ['<some-record-uuid>', {'_stats': {'views': 42, 'downloads': 420}}],
    ['<some-record-uuid>', {'_stats': {'views': 100, 'downloads': 200}}],
    ...
)