support elasticsearch version 7.x

[paularmand]

Trying intake-elasticsearch on ES7. Apparently ES now reports ['hits']['total'] back as a dict instead of a value. We would, or:

1. need to test the version of ES (not the library, but the server)
2. just try/catch it

The second approach leads to changing the following in _run_query:

        try:
            # elasticsearch >= version 7
            scroll_size = s['hits']['total']['value']
        except KeyError:
            # elasticsearch <= version 6 
            scroll_size = s['hits']['total']

Okay for a PR like so, or would you prefer not to support ES7 just yet? Or want a different approach?

ps: unsure what to do when s['hits']['total']['relation'] != 'eq' meaning that the total hits is not exact. We can force exact results by setting "track_total_hits": true, see https://www.elastic.co/guide/en/elasticsearch/reference/7.4/search-request-body.html#request-body-search-track-total-hits for more info. (This can be passed via the catalog qargs probably)

[martindurant]

If you know how to test the server version, that would work fine - otherwise the try/except would be OK as a workaround. I would be happy to see this in a PR.