Upgrade Stretchy for ES 2.x

[yozlet]

If/when we move to ElasticSearch 2.x, we'll need to upgrade Stretchy. And when we do that, we'll get a load of breakage (as I just discovered when doing an experimental bundle update):

• Stretchy.delete becomes Stretchy.delete_index (needed here)
• API#to_search no longer works - I think we might need API#request but there's more to it than that (needed here amongst other places)
• We can do some of the things we were doing in raw JSON now using the API (e.g. paging, field listing) but other things still need to be done in JSON (such as aggregations, for the stats feature)

CCing @agius, just in case he's able to give us any advice on this 🙏