Streaming/chunked API (and others) response

see ckan/ckan#1847, currently the response string is fully constructed before sending it, we could do something along the lines of using an iterator to send responses so we don't end up eating large chunks of memory when sending large api responses and perhaps sending these as chunked responses?

Review ckan/ckan#1847 first, then trace how large API responses are currently constructed and sent. Define completion as sending responses incrementally without retaining the full response in memory, including the proposed chunked behavior, and verify this across the affected APIs.

Written by the indexing model from the issue text.