Import in bulk instead of one by one

[dkarlovi]

Elasticsearch has a Bulk API. Using that in the importer command would mean we would do way less HTTP requests at a time, likely improving performance and robustness of the import process.

Since this package uses ongr Elasticsearch bundle to abstract interacting with ES, this would likely need to be supported there first.

Alternatively, this package could switch to Elastica which already supports bulk operations (but the switch would likely be quite a bit of work).

[dkarlovi]

This is invalid, it's already doing bulk upserts.