Elasticsearch queries should retry on failure

What does this MR do?

Reconfigure our elasticsearch integration so that failing requests will be retried, possibly against a different node, before giving up.

Are there points in the code the reviewer needs to double check?

Should we set reload_connections as well, per http://www.rubydoc.info/gems/elasticsearch-transport#Reloading_Hosts ? It shouldn't be needed for unicorn, since those processes are short-lived, but maybe it's worth it for sidekiq processes?

Why was this MR needed?

we should keep working when less-than-all elasticsearch nodes are available.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2681 (closed)