Stats Pusher to support array responses

[anjackson]

The JSON configuration of the stats pusher does not currently support responses that contain arrays. e.g. we could also query the CDX indexes directly (both crawl-time and access-time), like this:

http://cdx.api.wa.bl.uk/data-heritrix?sort=reverse&output=jsondict&limit=1&url=https://www.bl.uk/robots.txt

However, this returns an array, not a dictionary, and wa_stats can't be configured to deal with that.